5i8 THE POPULAR SCIENCE MONTHLY. 



servations 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 occu- 

 pied for tidal observations ; they can then be made with more preci- 

 sion 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 meteorolo- 

 gical relations, in order to complete deficient observations. Perma- 

 nent 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 con- 

 dition 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 featui-e upon which 

 the navigator relies, coast-line, out4ying dangers, peaks of mountains, 

 with their height, conspicuous objects, etc. Sea-charts are construct- 

 ed 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 pro- 

 jections, as it represents the meridians and parallels of latitude in 

 straight lines, thereby facilitating the laying down positions and bear- 

 ings. The careful hydrographer will plot his work on a conic pro- 

 jection, and thence transfer it to that of the Mercator. The gno- 

 monic projection projecting areas on a plane tangent to the earth 

 from the earth's centre represents the great circles, the shortest dis- 

 tances between two points by straight lines, and in this has advan- 

 tages for charts of entire oceans. As yet, this projection has not 

 been used to any extent. All conspicuous objects on which the navi- 

 gator depends should be given preference in distinctness of delinea- 

 tion over that of mere detail. 



Upon the intricacy of the configuration, especially that of danger- 

 ous 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 in- 

 terfere with clearness. Usually the work is first laid down on a scale 



