Popular Science Monthly/Volume 16/December 1879/Ocean Meteorology II
THE frequent examination of Maury's charts for the purpose of shortening tedious passages under sail, led to the idea of remodeling them for greater ease of consultation, and at the same time of adding the vast store of data accumulated since their publication.
The first conception of the new charts embraced only their salient features: from time to time, during the progress of the work, various details occurred and were added, so that to-day the undertaking may be said to be systematized, and it is this system which I shall describe.
The sources whence the information for the charts is derived, are two: log-books of ships of our own navy, and journals of merchant-vessels.
On board every vessel of the United States Navy it is obligatory to keep an official daily record, called the log-book. The first part contains full and explicit directions for keeping it; lists of the officers and men composing the ship's company; plans and sections of the ship; a description of the armament, boats, and small-arms; a table of deviations of the compasses; and a description of the meteorological instruments used, their location, and comparisons with standards. Following this matter are blank pages, suitably ruled, for a six months' record, two pages for each day; the left-hand page is chiefly for meteorological observations—the right, for miscellaneous events.
At the end of every hour, both day and night, and in port as well as at sea, the following items are observed by the midshipman of the watch, and recorded in their respective columns: the speed of the ship; direction and force of the wind; leeway; height of mercurial barometer and its attached thermometer; temperature of the air and of evaporation (dry-bulb and wet-bulb, both in a lattice-work case); temperature of the sea at the surface; weather by symbols; forms of clouds; portion of sky clear; condition of the sea; and the sail the ship is under. At the end of every four hours, the lieutenant in charge of the deck enters on the right-hand page such particulars of the weather as could not be described in the columns, together with whatever events occurred during his watch. Every day at sea, the navigator enters on the left-hand page the distance run since the preceding noon; the latitude and longitude at noon, both by observation and by account; the current (if any) experienced during the day; and the variation of the magnetic needle with the position in which it was determined.
The watches or tours of duty on board a vessel of war are divided into four-hour periods, each watch being in charge of a lieutenant, assisted by a midshipman; the number of observers throughout the twenty-four hours will, therefore, vary with the number of watch officers: generally there are four.
Each lieutenant is solely responsible for the correctness of the log during his watch; but, as different officers contribute to the record of a day, this lays the log-book open to both error and incongruity, if a general supervision were not exercised by some one person. Such is daily done by the navigator, who, after examination, certifies to its correctness, and then the commanding officer examines and approves it.
With accurate instruments, careful observers, and this system of scrutiny, there remains nothing to be desired in the way of a continuous, complete, and accurate record of the experience of a ship, whether cruising on the high-seas or at anchor in a landlocked harbor; and it is believed that more trustworthy observations are never taken at sea. Furthermore, they are made at such short intervals—every hour—and the atmospheric phenomena and corresponding instrumental changes are so closely contrasted side by side that no error, break, or flaw can enter, without easy detection.
I have been thus explicit regarding the log-books, in order that the accuracy of the charts which are based upon them may be fully appreciated.
As regards the data furnished by merchant-vessels, in 1878 a very complete meteorological journal was prepared at the Hydrographic Office for the use of ship-masters, and is issued to them free of charge either from the office directly, or from one of its agents in the principal commercial ports of the world. It is essentially like the log-book of the navy, and is for observations at sea only. When full, it is to be transmitted to Washington at the expense of the office. A number of sailing charts and all the latest hydrographic information are supplied gratis as an inducement to keep the journal. Hundreds of them are already afloat on ships of various nationalities, and are being filled with valuable data regarding every sea known to commercial enterprise.
Before proceeding to describe the method of compilation, I shall dwell for a moment on one of the items of record in the log-book, viz., the ascertainment of the ship's speed. Besides probably being of interest to many who yearly cross the sea in quest of either pleasure or health, a knowledge of this will tend to elucidate another matter of which I shall speak hereafter—the determination of whatever currents are drifting a ship, it may be, into serious danger.
The ship's speed is found by "heaving the log." The principle involved is the same as if one were to fasten the end of a tape-line, which is coiled on a spool, to a post, and then, holding the spool in his hand, he walked from the post at a uniform pace, allowing the line to easily roll off, but not become slack. If at the end of one minute he had walked 300 feet, in an hour he would have walked (at the same rate) 18,000 feet, or about three nautical miles.
Now, no stationary point exists in the ocean from which to measure, but this desideratum is attained by means of a thin flat board, sector shape, of eight inches' radius, and with the rounded edge loaded with lead to keep it upright in the water. Short lines connect the three corners of this "log-chip," as the sector is called, with the "log-line"—one of them by means of a wooden plug which is gently forced into a hole in a piece of wood fastened to the log-line about two feet from the chip. After well soaking and stretching, the log-line is marked as follows: A length of it about 100 feet from the chip is allowed for "stray-line," and then the length of a "knot" (for the sand-glass that runs for 28 seconds) is determined by this proportion. As the number of seconds in an hour is to the number of feet in a nautical mile, so is the length (in time) of the sand-glass to the length (in feet) of a knot; or 3600: 6086 28: 47·33.
The limit of stray-line from the log-chip is marked by a piece of red bunting six inches long, and each length of 47·33 feet after that by a piece of fish-line with one, two, three, etc., knots in it, according to its number from the limit of stray-line. Each length of 47·33 feet (the "knot") is subdivided into five equal parts, and a small piece of white bunting two inches long is turned into the line at every two-tenth division thus formed.
"To heave the log" is performed thus: one person holds the sand-glass, another the reel on which the log-line just described is coiled. and a third throws the log-chip with the line attached over the ship's stern; the chip, floating upright, is kept stationary by the resistance of the water, while the vessel moves on, and the line runs out; the midshipman watches it until the limit of stray-line just passes the rail, when he sharply says "Turn"; the glass is quickly reversed, the sand begins to run into the lower compartment, and both time and space are reckoned from the word turn. The "knots" reel off slowly or rapidly according to the ship's velocity, and, when the last grain of sand runs out, the line is instantly stopped. The number of knots and tenths run out denotes the speed at the moment of making the experiment, and, according to the conditions of wind, sea, and sail for the whole hour, the speed is deduced for the hour, and so entered in the columns.
To draw in the line, a quick, strong jerk on it frees the plug, when the chip floats horizontally, and can be hauled aboard with little resistance.
The "course steered," which is always a coördinate entry with the velocity, is obtained from a standard compass, whose every error is found and tabulated, to be applied when necessary. As the course and velocity are entered every hour in the log-book, we have thus a continuous record of each direction in which the ship headed, together with the distance she proceeded in that direction.
The courses and distances are the data by which, with the aid of a traverse-table, the ship's position may be found at any time—the position by "dead reckoning," or "account," as it is called. Independently of this, the position—"by observation"—is daily found by the navigator by altitudes of the sun, the moon, or the stars.
Suppose a ship to leave New York at noon of any day, and that her "run" is accurately kept until noon of the next day, when the latitude and longitude by account are found. The ship may not really be in this position: currents may have borne her along or athwart her course, yet we can not discover them; they act on log-chip and vessel alike: but let the position "by observation" be determined for the same instant that it is "by account," and we have at once a standard of comparison whereby the treacherous streams are made known. If none exist, the position by the two methods should agree within the small limit of error due to the unavoidable imperfection of both observers and instruments.
A third mode of ascertaining the ship's run is by the patent log—an instrument constantly towed astern at the end of a long line. It has a small propeller which the motion through the water causes to revolve. This revolution is communicated to a series of cogged wheels connected with hands that point to a circular scale—an arrangement not unlike a gas-meter. Every noon the log is hauled aboard, read, reset, and then thrown overboard, to record again the number of miles by which the ship nears her port. Being entirely independent of both dead reckoning and observation, it forms a kind of check on those two methods.
I will now enumerate some corroborating circumstances that must be considered in connection with the difference between the position by observation and that by account, ere this difference—its set and velocity—be tabulated as one of the permanent, ever-flowing currents of the ocean:
I. Temperature.—Of two contiguous bodies of water—one hot, the other cold—the latter, being specifically heavier, will displace the former, and hence a permanent current is established,
II. Evaporation.—Since no salts are taken up in the vapor, a body of salt water from which great evaporation takes place will be specifically heavier than an adjoining one that gives off less vapor, and so a continuous flow from the dense to the light fluid will be maintained.
III. Winds.—In a gale, the waves roll one after another in huge volumes toward the point to which the wind blows; and the friction of the wind upon the water produces a temporary surface set to leeward.
IV. Difference of Barometric Pressure.—In gales of wind, it is common for the barometer to fall from, say, 30·20 to 29·70—half an inch—in less than a day, and while the ship is passing over a comparatively small extent of ocean. Take a very extreme case, merely for illustration. Suppose two contiguous square miles of ocean, the barometer standing 30·20 over one of them, and 29·70 over the other. This difference of half an inch in the barometer is equivalent to a difference of about one quarter of a pound pressure per square inch of surface, or 36 pounds per square foot. Taking 6,086 feet as the side of a square mile, it will contain 37,039,396 square feet; each square foot sustains a difference of pressure of 36 pounds, so that there are in all 1,333,418,256 pounds more pressure on the square mile over which the barometer stands 30·20 than on the one over which it stands 29·70. It is evident that, in order to attain an equality of level, a very decided temporary set must take place from the former square mile toward the latter.
Instead of confining the case to the impossibly small area of two square miles, let us suppose a gradual fall of the barometer from one part of the ocean to the other—such a fall, in fact, over such an area as often comes within the experience of every naval officer—and it stands to reason that waves of the ocean, like those of the air, only smaller and more sluggish, are consequent upon every change of the barometer.
V. Rotation of the Earth.—From being at rest, suppose the earth to begin to revolve, as now, from west to east. On starting, the water of the ocean would, owing to its inertia, recede from the western shores of all the continents, and, as the earth continued to revolve, it would flow to the westward. For two reasons, however, it would be confined to equatorial regions: first, the centrifugal force there is greatest; and, second, the meridians converge as we near the poles.
This second reason will appear evident if we suppose a body of water of five degrees area and any depth to set out from the equator toward either pole. At each remove it would find the linear dimensions of a degree smaller. The depth remaining constant, its volume would be too great for an area of five degrees square in latitude 30°, still more so for one in latitude 60°, and so on. This constant crowding in extra-tropical zones would therefore constitute an opposing force sufficient to confine the flow of water to a zone where its volume would undergo little or no compression—that is, in the vicinity of the equator.
Arriving, then, at the eastern shores of the continents to the westward of those from which it started—at the North and South American shores, for instance, having started from Europe and Africa—and being banked up by constantly arriving volumes of water, it would be forced to the northward and to the southward along the coast-line of each continent; it would then flow to the eastward in high latitudes until reaching the western shores of the continents from which it started, where, owing to the divergence of the meridians toward the equator and the greater centrifugal force at that parallel, it would flow from the north and from the south along the shore-lines of the continents until reaching the intertropical zone, where it would again start westward on its circuit.
Imagine this system of circulation once set up, and nothing is more natural than that it should continue while the earth revolves; indeed, a glance at any current chart of the world will suffice to show the force of this reasoning.
It will now be seen how important a part the thermometer and hydrometer play in the discovery of oceanic currents: by the former a difference of temperature, and by the latter a difference of density, is quickly detected; and, if a decided difference of either kind is found, a permanent current may be fairly inferred. A consideration of the winds, whether an accidental gale, the constant trades, or the seasonal monsoon, may lead us to deduce intelligently whether a set that may have been experienced for days is a temporary surface-flow or a permanent current. So, also, keeping in view the range of the barometer for a few days—the locality and amount of its rise or fall—may assist in deciding whether a certain set be due to its extreme range or not. A consideration of the rotation of the earth is of assistance only in determining the general direction of the great ocean-currents.
A few other entries of the log-book require a passing notice. The direction of the wind is indicated by a vane in connection with a compass, and its force is estimated according to the following scale:
|Force of wind, nautical scale.||Nautical designation.||Sail that a full-rigged ship may carry, close-hauled by the wind; also her probable speed.||Sail that a full-rigged ship may carry, wind on quarter; also her probable speed.||Force of wind in pounds per square foot.||Velocity of wind in miles per hour.|
|O||Calm.||All sail.||All sail.||0||0|
|1||Light airs.||All plain sail and stay-sails; smooth sea; 0·5 to 1 knot per hour.||All plain sail and studding sails; smooth sea; 1 to 1·5 knot per hour.||0·064 to 0·019||1 to 2|
|2||Light breezes.||All plain sail and stay-sails; smooth sea; about 2 knots.||All plain sail and studding-sails; smooth sea; 2 to 3·5 knots.||0·08||4|
|3||Gentle beeezes.||All plain sail and stay-sails; smooth sea; 3 to 4 knots.||All plain sail and studding-sails; smooth sea; 4 to 5 knots.||0·36||9|
|4||Moderate breezes.||All plain sail and stay-sails; smooth sea; 5 to 6 knots.||All plain sail and studding-sails; smooth sea; 6 to 7 knots.||1·0||14|
|5||Stiff breezes.||Courses, top-sails; to’gallant sails, and stay-sails; mod. sea; 6 to 7 kts.||All plain sail and studding-sails; moderate sea; 8 to 9 knots.||1·5||17|
|6||Fresh breezes.||Courses, single-reefed top-sails, to'gallant sails; moderate sea; 7 to 9 knots.||Courses, top-sails, to'gallant sails, lower and topmast studding-sails; mod. sea; 10 to 12 kts.||2||20|
|7||Very fresh breezes.||Courses, double-reefed topsails, fore topmast stay-sail; moderate sea; about 7 knots.||Courses, single-reefed top-sails, to'gallant sails; moderate sea; 12 to 14 knots.||3||24|
|8||Moderate gale.||Single-reefed courses, treble-reefed fore and main top-sails, close-reefed, mizzen, fore top-mast stay-sail; rough sea; 4 to 5 knots.||Single-reefed courses, double-reefed fore and main top-sails, close-reefed mizzen; rough sea; about 10 knots.||5||30|
|9||Strong gale.||Close-reefed courses, close-reefed fore and main top-sails, storm stay-sail; rough sea.||Close-reefed courses, close-reefed fore and main top-sails, storm stay-sails; rough sea.||8||40|
|10||Very strong gale.||Close-reefed fore sail, close-reefed main top-sail, fore storm stay-sail; very rough sea.||Close-reefed fore-sail, close-reefed main top-sail, fore storm stay-sail; very rough sea.||23||67|
|11||Violent gale.||Storm-sails, or close-reefed main top-sail and fore storm stay-sail; very rough sea.||Close-reefed fore-sail, close-reefed main top-sail, fore storm stay-sail.||32||80|
|12||Hurricane, Typhoon, Cyclone.||None: lying to; drifting bodily to leeward.||Scudding under bare poles.||50 and upward||100 and upward|
The above sail and speed, corresponding to various forces of the wind, are but approximations to what really takes place according to particular circumstances, such as model of ship, course steered with reference to the wind, condition of the sea, etc.
The customary designations of the clouds are employed, and the portion of clear sky is denoted by figures—10 representing a wholly clear sky, 0 an entirely cloudy one, and intermediate numbers a sky partly clear and partly cloudy.
In the column descriptive of the weather, the following symbols are used, and by means of them, all the possible variations of weather that can occur in an hour, may be succinctly and accurately described:
|b.—Clear blue sky,|
|d.—Drizzling or light rain.|
|f.—Fog, or foggy weather.|
|g.—Gloomy, or dark, stormy-looking weather.|
|p.—Passing showers of rain.|
|r.—Rainy weather, or continuous rain.|
|s.—Snow, snowy weather, or snow falling.|
|u.—Ugly appearances, or threatening weather.|
|w.—Wet, or heavy dew.|
We now arrive at the method of compilation, and for this several blank forms are used which will be described as the necessity arises to speak of each kind.
First, however, every log-book and journal is closely examined for any errors it may contain, and, if such exist, they are scored with a red pencil, and hence do not enter into the compilation or charts.
Whatever judgment of the log-book this scrutiny warrants, is entered in it for guidance of the compiler.
The first blank to be used is Form A—a sheet 36 inches by 20. It is a projection according to Mercator of that part of the sphere between the 70th parallels of latitude north and south. Beginning with the equator and the meridian of Greenwich, parallels and meridians are drawn at every fifth degree, thus dividing the ocean, like a chess-board, into small squares. These are numbered consecutively from 1 to 1,667, so that, either by its number or its limits in latitude and longitude, the expanse of ocean covered by any square can be designated.
When a ship makes a passage under sail, she will cross certain of these squares on devious courses, and be in each square a short or a long period according as the winds and weather favor or oppose her. The particular square in each day can be ascertained by referring the noon position to Form A; and the hour of entering and of leaving it can be found by working the ship's run, by means of the courses and distances, backward or forward as necessary, from the noon position. Then, drawing heavy lines across the left-hand or meteorological page of the log-book at the hours of entry and of exit, it is evident that all the observations between these lines were taken in that square.
The limits of all the squares traversed are determined and marked in like manner, the number of each square is written between its bounding lines, and, when thus wholly prepared, the observations taken in each square during whatever length of time the ship was in it—whether one hour or several, provided it was continuous time in the same month—are compiled on one blank of Form B. This is a sheet of thick, durable paper, so ruled into columns with appropriate printed headings that each of the following-named items has a place for entry: the limits in latitude and longitude of the square, its number, and the number of hours the ship was in it; date of passing through the square, name of the ship, and the period covered by her log-book—all, that direct reference may be had at any time to the original sources. In a series of columns the thirty-two points of the compass are printed in regular order; blank spaces are provided on the right and left of each point—those on the left for the number expressing the wind's duration from that point, and those on the right for the figure denoting its mean force for the period of that duration. To compile the direction of the wind, the number of hours it was from the same point, whether consecutively or at different intervals, are counted, and the sum total is entered on the left opposite the point; for the force, the mean of the several hourly forces corresponding to the period of direction just mentioned is found and entered on the right of the point. Similarly for each point from which the wind blew while the ship was in the square.
As the entries regarding the wind in the log-book are hourly estimates of both its direction and force, for the hour—an interval that permits little variability in either quantity—accuracy to this degree is insured in both the compilation and charts. There is no averaging the force or direction for longer periods than an hour, and, as far as I am aware, this is the only system pursued by any nation wherein these items are compiled with such detail.
When calms or light, variable airs occur, the number of hours of each is counted, and the sum entered in its proper place. When a gale—that is, wind of a force of 8 and upward—happens, the number of hours it blew from any of the eight principal points of the compass (supposing it to have varied in direction), together with the force during each period, are duly tabulated.
The total number of hours of fog, of rain (including snow and mist), and of squalls (heavy, moderate, and light); the state of the weather by symbols; quantity of clear sky; and variation of the magnetic needle, with the location in which it was observed, are all entered in their respective places.
The mean of the mercurial barometer, attached thermometer, dry-bulb, wet-bulb, and temperature of the sea-water at the surface—that is, the mean of all the hourly observations of each of these quantities while in the square—is found and entered. Beside each is placed its daily range, by which is to be understood the mean of the différences between the daily maxima and minima for the number of days the ship was in the square.
Throughout a wide zone on both sides of the equator, the barometer has a remarkably regular oscillation, attaining two maxima and two minima every day. In order to discover the extent of this phenomenon, the hours of its recurrence, and the amplitude of the alternate rise and fall, the daily record of the barometer is carefully examined, and, whenever the phenomenon is found clearly defined, the hours of the two extreme lowest and two extreme highest readings, together with the readings themselves, are noted and tabulated on the blank. To save repetition, I will state here that all compilations of the barometer are reduced to 32° Fahr. and sea-level. As two more items of interest, the highest and the lowest readings of the barometer and dry-bulb that occurred during the whole time the ship was in the square are noted and entered. A miniature chart of a five-degree square, but on a sufficiently large scale to allow of considerable accuracy, is printed on the Form, for plotting the ship's track: the position of the ship being determined at 8 a. m., noon, and 8 p. m. of each day, these points afford the data for tracing the track. Whenever currents exist, their set in points and their velocity in knots and tenths of a knot per hour are written on this little chart at each noon position; an arrow is also projected from the track to indicate their direction. At every noon the temperature of the sea-water at the surface is very carefully taken, and entered on the chart on Form B, beside the noon position of the ship; it assists in coming to a conclusion as to the existence of currents.
This completes the tabulated portion of the compilation on Form B; but, to unite the whole, to trace the connection of the several related quantities, and to describe such matters as could not be otherwise noticed, ample remarks are made on the blank.
Thus, at a glance, are all the observations of a ship in each square visible on a single sheet; it is not assumed that what she experienced prevailed over all the expanse of the square; her track lies before us on the little chart that represents that square, and the tabulated observations beside it relate to that track only—a single line! If we have the observations and tracks of a large number of vessels, all over every part of a five-degree square for each month—in other words, if we have a multitude of sheets of Form B compiled—it is evident that we can easily deduce the meteorological features of that square, and judge whether they be like or dissimilar throughout its entire extent.
To follow up the fate of Form B, compilations having been made on many thousand sheets of it from all the log-books and journals that could be obtained relative to the expanse of ocean under consideration, the next step is to classify these, and to reduce the observations. The classification consists in grouping together all the sheets of the same square for the same month, without distinction as to year or ship; and, to reduce, all the observations on the blanks of each group are first transcribed to a single sheet of another blank. Form D—a kind of ledger, as it were, to which the separate sheets of Form B bear the relation of so many entries in a day-book.
It may not be entirely devoid of interest to some of my readers, to have here such a description of Form D as will enable them to reproduce it: a fac-simile before one would render the details of the reduction more easily intelligible.
At the top of the form are spaces for entering the number of the square and the name of the month. Under this are six vertical columns crossed by thirty-four horizontal lines. The headings of the columns in succession, from the observer's left to right, are as follows: 1st column, "Total No. of hours of wind from every alternate point"; 2d, "No. of hours of wind from every point"; 3d, "True direction of wind" (under this heading the points of the compass, beginning with north, are printed—one point on each line—down the first thirty-two lines of the blank, and "calms" and "variable winds" are on the last two lines ); 4th, "Mean force of wind from every point"; 5th, "Final mean force of wind from every alternate point"; and, 6th, "Percentage of wind from every alternate point."
To illustrate the use of Form D, suppose that for any square for any month—say No, 643 for July—there are fifty sheets of Form B. To collect the numerous observations of the wind scattered throughout these, each point is considered separately and in succession. The sum of the different periods that the wind was of the same force from the same point on all the sheets of Form B is found; the direction of the wind on this Form being magnetic, it is corrected for the variation of the compass, and then the sum is entered on Form D opposite that point which it becomes as a true direction.
Take a specific case, and let it be the magnetic north of Form B, with the variation one point westerly for the square under reduction: suppose that all the hours the wind was a force of 5 amounted to 80; then "80" would be entered in the 2d column, and "5" in the 4th column of Form D, on the horizontal line on which "N. by W." is printed; because a magnetic north wind becomes a true north-by-west wind when the variation is applied. Again, if all the hours the wind was of another force, say 3, from the same point (magnetic north) amounted to 120, then "120" would be placed in the 2d column, on the right of the previous sum 80, and "3" in the 4th column, on the right of the other force, 5, Similarly, with all the hours of each force, and with every point of the compass, the order being preserved throughout of having the first sum of hours in the 2d column correspond with the first force in the 4th column, the second sum of hours with the second force, and so on.
As the direction of the wind is given on the charts for only the alternate points, the observations of the intermediate points are distributed among those adjacent. Suppose the transfer of the observations of the sheets of Form B, for one point, to stand on Form D thus: 80 120 300 etc. . . . (hours). . . . N. by W. . . . 5; 3; 2; etc. . . . (Force). The point N. by W. not being used, all these observations must be distributed between North and North-northwest—half to each: 1. e., 40 hours, force 5; 60 hours, force 3; and 150 hours, force 2, must all be placed opposite North, and the same opposite N. N. W., just as if they had come from the sheets of Form B. The "Total No. of hours of wind from every alternate point" is now found by adding up the separate periods, and the sum is placed in the 1st column opposite its own point. To obtain the mean force for this sum, each separate period of hours is multiplied by the force peculiar to it, the products are added together, and their sum is divided by the "Total No. of hours of wind from every alternate point"; the quotient is the "Final mean force," which is accordingly placed in the 5th column. The totals of "Calms" and of "Variable winds" are now found, and carried out to the 1st column; then all the figures in this column are added up, and the result is the "Total No. of hours of winds, calms, and variables" observed in square No. 643 during the month of July. To complete the treatment of the winds, the percentage that the total number of hours from every alternate point, as well as the percentage that the calms and the variables are of the total number of hours of observation in the square, are worked out and entered in the 6th column.
On the back of Form D are columns for collecting the other data scattered over the sheets of Form B, viz.: 1. Barometer and thermometers. Beside the mean of each of these quantities transcribed from Form B is placed the number of hours of observation from which that mean is deduced, so that, in obtaining the final mean of the square for the month, due weight can be given to each individual mean. 2. The separate mean daily ranges of the preceding items, with the number of days of which the mean is formed written beside it; 3. The regular daily oscillations of the barometer; 4, The highest and the lowest readings of the barometer and dry-bulb, with the period during which each separate observation was made; 5. The total number of hours of fog, of rain, of squalls (heavy and light), and of gales from each quarter, with their strength; and, 6. A summary of the currents, weather, and magnetic variations. To complete Form D, a résumé of the remarks on the various sheets of Form B is made—a generalization of the experiences of the several ships—and transcribed to the back of the form. When the observations of every square for each month are collected on sheets of Form D in the manner described, the reduction of the data is complete for the part of the ocean under consideration. The final step is to arrange the matter in shape for the engraver and printer. This is done on sheets of Form C—a blank eight inches by ten—which is a facsimile of one of the squares of the monthly charts enlarged to a size suitable for writing all the data in the proper spaces. The data for all the squares for one month are transcribed from Form D to Form C; these are sent to the engraver; a proof-sheet comes back in due time; this is read and corrected; and eventually the charts themselves issue complete from the press. Until the year 1876 no systematic use was made of the log-books of our vessels of war. Every six months they were sent from each squadron to the Navy Department, where they have been accumulating since the days when Maury had the supply then on hand compiled for his charts.
Now, a large number await examination—a rich mine of valuable material. A set of charts for the Pacific Ocean between the equator and latitude 45° north, and from the American coast to the 180th meridian, are the first fruits of labor in this mine. The complete set consists of fifteen different sheets: 1. Twelve on which the information peculiar to each month is given on a separate sheet; 2. One that summarizes all the observations of the direction and force of the wind of every month and year; 3. One that similarly gives all the observations of the barometer, thermometer, and weather; and, 4. One in colors showing the wind systems, stormy regions, and areas of equal heat and equal pressure—a graphical exhibit of what the other charts contain.
Preceding the set are, a table giving for every month the latitude and longitude in which each ship entered or lost the trade-winds; a list of the log-books whose data enter into the charts; and a preface chiefly descriptive of the method of compilation. All are printed on thick paper and bound together in a folio volume convenient for use.
The method of compilation having been described, the charts will now be easily understood.
The figure on page 191 is a facsimile of two adjoining squares of a monthly sheet—February. They cover the small area of ocean west of the Mexican coast between the parallels of 15° and 20° north and the meridians of 95° and 105° west. The month and square constitute jointly the unit for which all the information is given; for some squares this is meager, for others full—always dependent on the length of time ships were in the square and the number of log-books examined for it.
The explanation of one of the above squares—No. 106—will afford a key to the whole series. The figures inclosed by circles relate to the winds; those between the outer and the middle circle indicate its duration and force; and those between the middle and the inner circle, its percentage, from every alternate point. The points are inclosed by parts of radii that extend from the outer to the inner circle: thus, the two radii opening toward the upper left-hand corner, and containing the figures 20 inclose the N. W. point; the two containing 8 inclose the N. N. W. point, and so on to the right through North, N. N. E., etc. If we add together the hairline figures between the outer and the middle circle, as 226, 85, 76, etc., they will amount to 1,062; add to this the figures under the heading "calm" (38), and "var." (wind) (10), and the total is 1,110. This means that if the hours all the vessels spent in the square were added together, the total would be 1,110 hours, or 46 days 6 hours; this is the whole period of observation in this square for this month; it is composed of fragments of February collected from many years. An hour is the unit of observation, and a vessel had to be a whole hour in a square in order to have it constitute an observation. In this square, then, there were 1,110 observations of the wind: of this number, it was 226 hours from the N. W.; 85 hours from the N. N. W.; 76 hours from North, etc. The small dark figures to the right of the hair-line figures indicate the force of the wind: thus, the 3 annexed to 226 signifies that the mean force for those 226 hours was 3, according to the scale already given. The number of hours of wind from any one point may have been the experience of one or several vessels; and that number may be composed of hours of light, gentle, fresh, and strong breezes; but, however such variations may have occurred, the mean force is indicated by the small dark figures.
It would, no doubt, be desirable to give the percentage of different forces of the wind from each point, but this would greatly detract from the simplicity and ease of consultation of the charts. The defect is, however, approximately met by giving, as is done, the extreme variations of the wind, i. e., the number of calms as well as of heavy squalls and gales. The figures between the middle and the inner circle denote the percentage of wind from every alternate point: thus, of the 1,110 hours of observation, the wind was 20 per cent, of the time from the N. W.; 8 per cent, from N. N. W.; 3 per cent, was calm; and one per cent, light variable winds. The little table below the number of the square is to be read thus: of the 1,110 hours, 30 were calm; 10 were characterized by light airs flying all round the compass; no fog; 9 hours were rainy; no heavy squalls; and no light squalls. In the upper left-hand corner is the mean of the barometer, 30·05—the mean of all the hourly observations taken in the square during the month; under it is the mean daily range, 0·09 inch—the mean of the differences between the daily maxima and minima for 46 days. In the upper right-hand corner, under D. B. (dry-bulb) is the mean temperature of the air (in the shade), 80° Fahr.; the 5° under it is the mean daily range, both means obtained as for the barometer. Similarly, in the lower right-hand corner, is the mean temperature of evaporation and its daily range; and, in the lower left-hand corner, the mean temperature of the sea-water at the surface and its mean daily range.
The remarks within the circles are drawn from the experience of all the vessels that passed through the square in all the months of the year. It is their aim to fill up the outline character of the square afforded by the figures. While these remarks run through all the squares that have sufficiently similar features to be described together, the figures are applicable only to the square and month in which they appear. Thus are the climatic features of each small area of ocean delineated.
It should be understood that these charts merely exhibit the experience of the past reduced to a scale of probabilities for the future: if, then, the conditions stated in any square be not exactly realized, the system should not hence be condemned.
Let any navigator consider the degree of dependence he will place on a trustworthy record of a passage he once made; the confidence with which he will refer to that record—to the experience of a single voyage!—for his guidance in traversing the same ground again: now, it is not the record of one passage alone, but of many—all conveniently arranged, with every discernible error and inaccuracy eliminated—that is given in any square of these charts.
As a fair way of regarding them, let us consider the data of the square 106: of the 1,110 observations of the wind, it was 226 hours, or 20 per cent, of the whole time, from the N. W.; hence, at any future time, there are twenty chances for a K W. wind of a force of 3, against eight for a K N. W. wind, force 2; against seven for a N. wind, force 3; against three for calms and one for very light airs.
The observations of the direction of the wind on Maury's pilot charts have been incorporated with those extracted from log-books and journals of recent date, and, although they outnumber the latter, still they do not form the basis of the charts. That basis is the variety of observations compiled and reduced in the careful and laborious manner already described, and which have then been examined from every standpoint to elicit whatever beam of information they might contain to illumine the ocean highways. Therefore, in justice to the amount of thought, care, and labor bestowed on these charts, it must be stated that they are not a rearrangement of old matter, but are essentially new, and from original sources.
The second series of charts (now in course of compilation) embraces a series for the whole Atlantic between the parallels of 60° north and 60° south. The observations of the direction of the wind on Maury's pilot-charts, to the number of about 2,600,000, or 300 years, will be embodied in them: the actual period over which these observations extend is from the year 1800 to 1855. But, as with the Pacific charts, the real groundwork of the Atlantic series is the various kinds of observations at present compiled in the careful manner already described from recent log-books and meteorological journals. In number, these observations will amount to about 650,000 hours, or 75 years. The actual period over which they will extend is from the year 1855 to 1881, the time when it is confidently hoped this set will be ready for issue. By far the greater part of the compilation is already done. The Atlantic charts, then, will contain hourly observations to the number of about 3,250,000, or nearly 375 years: in other words, if it were imposed on a single ship to collect this mass of data, she would have to cruise in all parts of the Atlantic during every month of the year, for a period of 375 years, without ever going into port!
As, however, these observations were collected by a multitude of vessels and during a continuous series of 81 years, several vessels were observing at the same time in different parts of the ocean. Surely, this is an abundance of facts that must render indisputable the information contained in the more frequented squares: more would be mere accumulation, without perceptibly affecting the mean results.
On some accounts it would be desirable to have the areas for which the information is classified smaller than 5° squares, as 1° squares, for example; but, again, there are objections, all but insuperable, to such a system:
1. To collect data for it sufficient to give trustworthy results, would require a fleet of cruisers almost as large as the combined merchant marine of the world—all to be assiduously engaged for many years. This is unattainable. Even with the inducements now offered, and notwithstanding that the undertaking is mainly for their benefit, only a very small percentage of all the masters of merchant-vessels will take the trouble to keep a meteorological journal with the requisite accuracy and care. Were it not for the excellent log-books of our ships of war, our knowledge of the phenomena of the ocean would indeed be most meager and inaccurate.
2. Supposing the data obtainable, the organization essential to dealing with it in such detail would be immense: a lifetime would hardly suffice to reach a practical result,
3. Its publication would require five times as many volumes as the present system—a series already bulky enough to deter any one from increasing it.
4. Throughout by far the greater part of the ocean, the several quantities do not vary rapidly enough to warrant compiling them for such small areas.
The prevailing direction and force of the wind are substantially the same in many adjoining 1° squares; and so also are the pressure and temperature of the air, the weather, etc.: therefore, to classify them for 1° squares would only be multiplying what, for the most part, was equally applicable to the whole extent of a 5° square.
Besides, the object sought by the 1° system, the determination of the well-defined limits of the different phenomena, is more accurately attained by a method pursued in connection with the 5° system: on a Mercator's projection of very large scale, all the observations relating to one subject of inquiry are plotted in the position where they were observed. For example, the winds: at each noon position of the ship, an arrow is drawn to indicate its direction, and a small figure placed beside it to denote the force; a circle represents calms, and several short lines radiating from a point, light variable airs.
This continued until the whole sheet is studded with symbols, it is evident that we can determine, not to the closeness of one degree only, but to within a few miles, the precise area covered by the trades, or calms, or monsoons, or irregular breezes. The observations of the temperature of the air, of the sea-water, and of the barometer, are all similarly plotted, each on a sheet by itself. In every instance the symbols are in different colors, to distinguish the data peculiar to each month.
In order to determine with the greatest possible precision the limits of the Gulf Stream, as well as the veins of varied temperature that permeate it, a separate sheet for each month on an unusually large scale is provided for the observations relating to it.
It might seem that, instead of publishing such elaborate charts as the series described, a single sheet containing merely the conclusions arrived at would suffice—a chart showing those courses from port to port on which the most favorable winds and weather would be found. As well lay railway-tracks over the ocean and expect ships to glide upon them! In matters pertaining to their profession, none are more tenacious of their opinions than sailors—and justly so: they form them after hard experience. To dislodge those opinions it must be proved wherein they are faulty and others correct; and this can not be done by mere results. To lay down a rigid rule for a man to follow is to deprive him of the exercise of discretion and judgment—qualities in which it is eminently proper a sailor should be untrammeled. No, a track-chart is a useful auxiliary—it partly solves the problem of tracing the best course from port to port, and such a chart will eventually form part of the entire set; but a full exhibition of all the data on which the judgment is based is essential to every intelligent seaman.
It is the log-books of ships of our own navy from 1855 to 1877—the large accumulation of twenty-two years—that are now undergoing compilation at the Hydrographic Office for the series of charts described: since 1877, by an order of the Navy Department, the compilation is made by the navigator of each cruising ship. Being an officer of many years' experience at sea, and having direct and daily supervision of the log-book, there is great advantage in having the data arranged in the requisite form, on the spot and at the time of its occurrence, by such a competent person.
Both the observations and compilations are made with a definite object in view, and, as that is to furnish charts for their future guidance, it is an incentive to the officers engaged in their preparation to make them as trustworthy as possible. The compilation is to continue until charts for all the frequented portions of every ocean are published.
When a log-book is full, both it and the compilations are sent to Washington, where they are examined, compared, and used as found necessary.
In an article in a former number of this magazine, I have said that it is impossible to predict, as is done on the land, what the weather will be in various parts of the ocean for any short period; there we lack the stationary points of observation with direct and instant communication: as pointed out by Maury, the most that can be done in this way is to warn European countries by telegraph of the approach of storms that traverse the Atlantic from the American Continent; and of late this has been successfully done by the "New York Herald."
In conclusion, I will merely allude to the utility of the charts that form the subject of this article. If, on land, it be optional to choose one's residence according to the salubrity of the climate, so at sea, the mariner, with a panorama of the winds and weather spread before him, can direct his course through only those squares that are favorable and avoid the stormy.
Moreover, the novice to the sea or the philosopher in his study can, by a mere inspection of them, see what has passed over the waste of waters during the last hundred years, and be more fully and accurately informed regarding what in all probability he would have to encounter, in the way of aerial phenomena in an ocean-voyage, than the most weather-beaten tar that plows the main.