Page:Popular Science Monthly Volume 8.djvu/536

518 for a shorter time, and by rough means, may prove of some value, and such should be made daily.

Meteorological observations, the direction and force of the winds, the appearance of the sky and clouds, temperature, the pressure and humidity of the atmosphere, etc., should be made at the stations occupied for tidal observations; they can then be made with more precision than those usually made on board ship.

Every opportunity should be availed of for gathering information from intelligent residents in regard to the local, tidal, and meteorological relations, in order to complete deficient observations. Permanent currents are correctly ascertained in places where a vessel can anchor, by various methods of observation, on the deep sea generally by the difference between the position by observation and that by the dead-reckoning.

The active hydrographic surveyor will not, while on the ocean, neglect to aid in the labor of the physicist, by examining into the condition of the water, its temperature at the surface and at various depths, its specific gravity and salinity, its fauna and flora, and by contributing to the natural sciences, general geography, geology, and ethnology, while in regions which may be not at all, or but little, explored.

The hydrographic part of the information thus obtained is laid down for the use of the navigator in charts and text-books in such a manner as to be rendered complete without interfering with clearness and ready comprehension.

Charts must contain with distinctness every feature upon which the navigator relies, coast-line, outlying dangers, peaks of mountains, with their height, conspicuous objects, etc. Sea-charts are constructed for publication on Mercator's projection, although this projection distorts the relative size of the several areas and the bearings of points; the more so the farther the chart is extended toward the poles. Navigators, however, prefer it to the more correct conic projections, as it represents the meridians and parallels of latitude in straight lines, thereby facilitating the laying down positions and bearings. The careful hydrographer will plot his work on a conic projection, and thence transfer it to that of the Mercator. The gnomonic projection—projecting areas on a plane tangent to the earth from the earth's centre—represents the great circles, the shortest distances between two points by straight lines, and in this has advantages for charts of entire oceans. As yet, this projection has not been used to any extent. All conspicuous objects on which the navigator depends should be given preference in distinctness of delineation over that of mere detail.

Upon the intricacy of the configuration, especially that of dangerous passages, will depend the scale to be adopted, which should not be so large as to render the chart unhandy, and not so small as to interfere with clearness. Usually the work is first laid down on a scale