Popular Science Moiilhlij
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��representing; depths spaced more or less widely apart, according to the depth and nature of the bottom. If the depths are great and the bottom of sand or mud, the lines and soundings are wide apart. If the depths arc not great antl the bot- tom rocky and broken, the soundings are closer together; but there is always a considerable interval between liic lines of soundings and between the individual soundings. The soundings represent only the depth over a space of a few inches where the lead touclied bottom. It is between the scnindings that the danger may lie.
Thus, in the closest survey, large spaces are left, over which the depths are not absolutely known. Jagged pinnacles of rock projecting from the bottom may rip open the plates of a pass ing vessel. The project' masts of a sunken wreck may be a menace the navigator, al though not visible above the surface. The lead may slide off a precipitous rock and give no indication of the true depth. A line of soundings has but one dimension, length. The wire- drag line has two dimensions, length and breadth. For every mile of distance dragged every danger in a square mile of area is detected with absolute certaint\'.
Witii the lead line their discovery is more or less a chance, and it is difficult and often well nigh iniiiossible, to find a rock or shoal of small extent even when its approxi- mate position is known. The vessel searching for it is as apt to run against the obstruction as to find it by sounding. With the drag such a danger cannot escape. Hence it is the only means of finding all submerged dangers in certain areas. Safety of navigation can be assured by no other means.
���The hoisting and measuring equipment on the towing launch
��The wire-drag is operated in the fol- lowing manner: A horizontal wire sup- ported at any desired dejjlh in the water l)y a system of uprights attached to floats at the surface and held down by weights, is drawn through the water by power boats. Any rock or shoal pro- jecting from the bottom above the effective depth at which the drag is set is caught by the wire. Soundings are then taken over the spot, and its position is located by angles taken to previously (lelermined i^oints on shore. The sound- ings are afterward plotted and placed upon the charts. In practice the drag has developed into a somewhat compli- cated mechanism, but in emergencies a simple form of drag ma\' be readily improvised. Modified forms of the tlrag have been used for find- and removing mines, id for locating sunken wrecks and buoys. It is obviously adapted to many such uses.
The a\-crage cost of a w i r e - d r a g party is thirty thousand dollars, based on a season's work of from six to eight months. The cost per day is about two hundred and fift}' dollars. Some idea of the importance to this country of surveys of its coasts may be gained by recalling to mind that the coast line of the United States and Alaska, measured along its general trend, ex- ceeds e!e\cn thousand five h mid red miles in length. To represent the actual shore line which must be surveyed and which includes all the islands, ba\'s, sounds, and rivers in the tidal belt, these figures reach the large total of ninety-one thou.sand miles; and to this must be added the shore line of Forto Rico.Ciuam.Tutuila, the Hawaiian and the Philippine Islands, whose coast line is twehe thousand statute miles.
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