Page:Popular Science Monthly Volume 8.djvu/537

Rh 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 soundings obtained, especially in harbors, will be far too numerous to represent 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 obtained. 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 sanding (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 harbors and special coast-charts the compass-points are generally laid off 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 off from the true north.

General charts, and frequently 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 position 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