[Illustration: FIG. 15. THE SIGSBEE SOUNDING MACHINE ON A SURVEYING VESSEL.] [Illustration: FIG. 16. LONGITUDINAL SECTION OF SURVEYING STEAMER _FATHOMER_, SHOWING GENERAL ARRANGEMENTS.] The offshore soundings are made from a surveying steamer; the inshore work is usually done by a launch or small boat. So far as the navigational use of charts is concerned it is important that the hydrography shall show the limiting depths and the freedom from dangers, of channels, entrances, harbors, and anchorages. It is also desirable that the soundings shall be carried off shore at least as far as the one-hundred-fathom curve, as with the modern forms of navigational sounding machines it is possible for vessels under way to obtain soundings to this depth, and such soundings may be of value in identifying the location of the vessel. For depths greater than one hundred fathoms the soundings have less direct value to navigation except as proving the absence of shoaler areas, but soundings throughout the oceanic regions are of great geographical interest as well as of direct practical value in the laying of cables. It is obvious that the plan of mapping the sea bottom by dropping a lead at intervals over its hidden surface is far from an ideal one. The lead gives the depth only at the point at which it touches the bottom, and no information as to the space between the casts except such as may be inferred from the relation of successive soundings. In numerous cases, after what was considered a very thorough survey of a region had been made, at some later day a pinnacle rock or other danger has been discovered. For instance, a very detailed hydrographic survey of Buzzards Bay was made in 1895; the sounding lines were run at intervals of 50 to 100 yards, and 91,000 soundings were made for a single sheet. Within this area the cruiser _Brooklyn_ in 1902 touched a rock which was found to have 18 feet over it. (Fig. 17.) The least depth in the vicinity developed in the original survey was 31 feet. For the satisfactory development of hydrographic work some invention is much needed which as it passes along the bottom will give a continuous depth curve. Several devices have successfully accomplished this in shoal water, but great credit awaits the inventor who designs something of more general application. [Illustration: FIG. 17. PORTION OF ORIGINAL HYDROGRAPHIC SHEET, BUZZARDS BAY, ON SCALE 1-10000, SHOWING AREA CLOSELY SOUNDED IN 1895, WHERE THE _BROOKLYN_ STRUCK IN 1902.]