Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . isbelieved, equally as satisfactory as, that employed by theMississippi River Commission, in which a series of weightsare employed to give a proper uniform tension. The chiefobject to be attained in tension is steadiness and uniformityof tension ; the simplest device which will attain this end isnaturally the best. Two general forms of such devices areemployed by the U. S. Geological Survey, one for measure-ment of base lines along railways, where the surf
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . isbelieved, equally as satisfactory as, that employed by theMississippi River Commission, in which a series of weightsare employed to give a proper uniform tension. The chiefobject to be attained in tension is steadiness and uniformityof tension ; the simplest device which will attain this end isnaturally the best. Two general forms of such devices areemployed by the U. S. Geological Survey, one for measure-ment of base lines along railways, where the surface of thetics or the roadbed furnishes support for the tape, and thedevice must therefore be of such kind as to permit of theends being brought close to the surface; the other is em-ployed in measure made over rough ground, where the tapemay frequently be raised to considerable heights above thesurface and be supported on pegs. The stretcher used by the Geological Survey for measur-ing on railways is illustrated in Fig. 153, and was devisedby Mr. H. L. Baldwin. It consists of an ordinarj spring*^ 504 FIELD-WORK OF BASE MEASUREMENT,. LAYING OUT THE BASE. 5^5 balance attached to the forward end of the tape, where atension of twenty pounds is appHed, the rear end of the tapebeing caught over a hook which is held steadily by a longscrew with a wing-nut, by which the zero of the tape may beexactly adjusted over the mark scratched on the zinc spring-balance is held by a wire running over a wheel,which latter is worked by a lever and held by ratchets in anydesired position, so that by turning the wheel a uniformstrain is placed on the spring-balance, which is held at thedesired tension by the ratchets. The tape-stretcher used by the U. S. Geological Surveyoff railways consists of a board about 5 feet long, to the for-ward end of which is attached by astrong hinge a wooden lever about 5feet in length, through the larger por-tion of the length of which is a slot(Fig. 154). Th
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