HYDR GRAPHY. 5 1 9 



large enough to show at a glance any fault in the projection, and then 

 reduced to the scale decided on for publication. On the latter, objects 

 of importance, especially dangers to navigation, should be exaggerated 

 in preference to their not being sufficiently conspicuous. The sound- 

 ings obtained, especially in harbors, will be far too numerous to repre- 

 sent them all, even upon the working-sheet ; care must be taken in 

 selecting the characteristic soundings, which must be reduced to a 

 certain state of the tide, usually to low water, and they must be 

 placed on the exact spot representing that in which they were ob- 

 tained. Heretofore these were expressed in the standard measure of 

 the country in which the chart was published, but recently the French 

 metre has been adopted by all maritime nations, excepting Great 

 Britain and the United States, who use the English fathom and foot. 

 It is preferable to use on the same chart but one unit, either fathoms 

 or feet, as the use of both, even with the shading, frequently leads to 

 error. In order to show better the structure of the bottom, and to 

 make irregularities more conspicuous, curves of equal depths fathom- 

 curves are laid down. The denomination of the curves depends upon 

 the depth of water that can be carried into the harbor or along the 

 coast. Harbor charts generally show the five, three, two, and one 

 fathom curves, the latter three often distinguished by shades of sand- 

 ing (dots to represent sand) ; the five-fathom curve is expressed by 

 rows of five dots on the line of the curve. Coast-charts generally 

 show in addition a ten, fifty, and one-hundred fathom curve. 



The character of the bottom is represented by the first letter, or an 

 abbreviation of the word, expressing it ; currents by arrows, with the 

 force in knots per hour or per day placed along them ; buoys and 

 beacons are shown by conventional signs. 



Lines of bearing point out the courses to be steered, and guide 

 also in avoiding dangers. Views, placed so as not to interfere with 

 the sailing-ground, show the appearance of the land on the bearings 

 on which they are taken. 



An important feature of the chart is the compass, placed in such 

 positions as are most convenient for taking off the courses. On har- 

 bors and special coast-charts the compass-points are generally laid ofi 

 from the magnetic north line; on general ocean-charts, on which the 

 variation changes rapidly with the lateral distances from the direction 

 of the magnetic curves, they are laid oif from the true north. 



General charts, and fi-equently harbor-charts, have the projection 

 drawn over them, from which the latitude and longitude of any point 

 represented on it can be ascertained minutely; where the projection is 

 not thus drawn, the astronomical position of a well-defined point is 

 given, usually under the title, with the mention of the primary posi- 

 tion to which it has been referred. The title also embraces the tidal 

 hour, with rise and fall of tide, at the full and change ; the unit of 

 measure in which soundings and elevations are expressed ; the scale 



