COAST SURVEY 761 10 feet long, 5 inches wide, and 1J inch thick, so graduated that the number of divisions upon it, as seen between the upper and lower hori- zontal wires of the telescope of the alhidade, is equal to the number of units in the distance between the observer's eye and the rod held at right angles to the line of sight. In all cases it should be graduated experimentally for the particular instrument and eye of the observer who has it in use. Thus the topographer, with- out moving from his position, can determine with accuracy the distances from him of any number of points within 300 metres. The tel- emeter has given very satisfactory results. Hydrography. The survey of the land having been completed by the method detailed, the hydrographer is ready to plot his soundings and observations in their proper places on the map. Under the head of hydrography are placed all those operations which are per- formed at sea for the determinations of the positions of the rocks and shoals, the depths of the water, and the investigations of tides, cur- rents, &c. The principal labor is that of sound- ing. With the outlines of the shores furnished by the topographer, the hydrographer has only to measure certain angles upon the shore sig- nals at regular intervals in the progress of a line of soundings, and the position of his boat or vessel on the line is thus determined, and of necessity the soundings taken on the line. These determinations are so accurate that the navigator may run his course by the soundings given on his charts. The angles are measured by a sextant, and the positions plotted upon the chart. In practice, the position of the boat is determined at intervals which vary ac- cording to the depth of water, the speed of the boat, the force of the tidal current, the fre- quency with which the soundings are taken, and the character of the bottom. Thus in strong tideway, or in a rocky and uneven bot- tom, angles would be taken at very short in- tervals ; while in smooth even bottom as much as five minutes is often allowed to elapse between any two angles. The intermediate soundings, being taken at equal intervals of time, are laid down at equal intervals between Ihe positions determined by angles. The num- ber of soundings is generally so great that the features of the bottom of the sea, within a wide belt adjacent to the coast, are almost as well determined as those of the land ; and with the further assistance of the specimens of the bot- tom which are brought up by the sounding-lead, the mariner may during foggy weather deter- mine his position with considerable accuracy by soundings alone. Physical Hydrography. The hydrographer determines the soundings along the coast and in the various harbors, the position and extent of the various shoals and rocks and the depth of water upon them, the rise and fall of the tides, the direction and ve- Tocity of the currents, and the various changes that take place in the channels and bars of our coast. Physical hydrography has for its prov- ince the investigation of the causes of these changes, with a view to determining the laws upon which they depend, and if possible to provide remedies. It studies in particular the whole subject of tidal currents, and their effect upon the harbor channels ; the effect upon these channels of the increase of wharf lines in cities ; the improvement of harbors by break- waters and sea walls ; the action of the sea upon beaches; and all of the many questions which arise in the discussion of the laws of change in harbors and on the coast. The coast survey has done much in the way of in- vestigations in this science, especially in the harbors of Boston and Portland. In the latter city extensive operations are in progress for improving the Back bay, the result of the rec- ommendations of the present superintendent, based upon surveys and reports made by his officers. In Boston a most extensive system of improvements has been going on for years, which will in a short time greatly increase the value of that already excellent harbor. At Edgartown and Vineyard Haven in Massa- chusetts, at the harbor of Nantucket, and at Chatham, examinations have been made and improvements projected, which will ultimately redound greatly to the advantage of those har- bors. Constant observations are made in New York harbor and its vicinity, and such improve- ments suggested as will arrest any unfavorable changes in the channel. This is a work of great labor and importance, and is increasing in success and usefulness year by year. Tides. One of the most difficult questions presented to men of science is the solution of the problem of the tides. The importance to the mariner of knowing the exact rise and fall, and the times of high and low water, may be appre- ciated when we consider that the entrances to many rivers and harbors on the Atlantic coast are obstructed by sand bars, which can only be crossed by vessels of more than ordinary draught at the highest stage of the water. It becomes therefore an object of immense im- portance to commerce and navigation to deter- mine the laws which govern this rising and falling of the sea, and to furnish such rules as will enable the mariner to predict with cer- tainty all the phenomena connected with this subject. There is indeed no subject which is so constantly in a sailor's thoughts, on ap- proaching the entrance to harbors and rivers, as the tides. In many cases the safety of his ship and his life depend upon his precise knowledge of the time of high water, or the ebb and flow of the current. The theoretical solution of this question was one of the first re- sults of Newton's theory of gravitation. The most casual observer may notice the close de- pendence of the tidal phenomena upon the moon's motion. The wave of high water fol- lows the moon in her motion around the earth by a determinate interval of time; and the height of the tide depends also upon the posi- tion of the sun and moon. As the elevation of
