. Handbook of railroad construction; for the use of American engineers. Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States .. . 35. If we wish to go from A to B, fig. 12 (a), we should of course Fig. 12 (a). take first thestraight line;but beingobliged toavoid the hillC, on arriv-ing at df, weshould nottry to recoverthat line at e, but proceed at once to B. Also as we are obliged to passthrough d, we ought to go directly to d and not by the wayof c; and the same idea is repeated between A and
. Handbook of railroad construction; for the use of American engineers. Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States .. . 35. If we wish to go from A to B, fig. 12 (a), we should of course Fig. 12 (a). take first thestraight line;but beingobliged toavoid the hillC, on arriv-ing at df, weshould nottry to recoverthat line at e, but proceed at once to B. Also as we are obliged to passthrough d, we ought to go directly to d and not by the wayof c; and the same idea is repeated between A and fZ B. Few rules can be given in thechoice of routes. Practice only will enable the engineer tofind the best location for a railroad. 2*. 18 HANDBOOK OF RAILROAD CONSTRUCTION. BAROMETRICAL LEVELLING. 36. The relative height of summits, the rate of fall ofstreams, and absolute elevation, within a few feet, maybe easily, rapidly, and cheaply found by the also affords an excellent check upon subsequent lev-elling operations. The results thus obtained depend uponthe physical property, that the density of the air decreasesas the square of the height. 37. The barometer is a glass tube, partly filled with mer-cury, having a vacuum in the upper part. By it the exactdensity of the air at any point is determined. Accompany-ing are two thermometers; one attached, showing the tem-perature of the barometer; the other detached, showing theatmospheric temperature. 38. Knowing now the manner of finding the density ofthe air at any two points, and also the relation betweendensity and height, the operation of levelling by the barom-eter is very simple. The modus operandi is as follows, (see tables A, B, C,and D): — Let u
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