Page:Popular Science Monthly Volume 16.djvu/194

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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