Page:Popular Science Monthly Volume 8.djvu/534

516 of which the earlier are more or less rough ad unconnected, and even some of later dates cannot be entirely relied upon.

Running surveys, more or less in detail, are generally the precursors of the more strict geodetic survey, but, in order to answer the wants of navigation, these should always be based upon a triangulation between natural landmarks, checked at reasonable distances by Very careful shore observations for latitude and longitude, and the latter carried directly from a central position to the most prominent points of the thus surveyed area and back again, and the central position connected in the same manner with the nearest satisfactorily determined position, to which the longitudes of that locality are generally referred.

The telegraph-cables which already connect many of the most important places will soon gird the globe in several belts, and will afford the means for ascertaining great meridional distances with almost absolute correctness. There will thus be furnished a great number of primary positions from which the longitude may be carried in coördinate lines to secondary places. In this manner a network of points spread over the globe will be attained, corresponding to the primary and second triangle points of great geodetical operations.

The completeness and correctness of a running survey depend upon the time devoted to it and the difficulties encountered; frequently the coast-line is only traced in from point to point, or from the shore-ends of the lines of soundings by the eye; the points of land, however, especially the salient ones, should always be fixed by angles to or from the established landmarks, as should also all outlying dangers and all features bearing directly upon or assisting navigation.

The surveys of harbors and anchorages should be as complete as possible; if time permits, beacons should be erected for triangulation, and the plane-table employed for obtaining the shore-line. The parts of the latter which are merely traced in approximately should be distinguished on the chart by a broken line.

The soundings should always be numerous enough to show the configuration of the bottom of harbors, and off a seacoast the gradual rise from great depths to the shore, islands, and banks, so that the characteristic curves of the depths may be shown with precision on the charts; for harbors generally the one, two, three, and five fathom curves are marked; on coast-charts, those of three, five, ten, twenty, fifty, and one hundred fathoms.

When sounding from a vessel in motion or from a boat, the lead should be tried at intervals, even when it is anticipated that the bottom will not be readied, not only on account of the possibility of the discovery of a sudden elevation, but for the purpose of placing the negative soundings on the chart, which show conclusively the absence of danger and that the ground has been examined.

For such negative soundings, as much line should be used as the