Wentworth, G. A. (George Albert), 1835-1906 · Boston, Ginn & company · leaf 234
THE Y LEVEL. LEVELLING FOR SECTION. 61 To find the difference of level of two places, one of which cannotbe seen from the other, and both invisible from the same place; or,when the two places differ considerably in level, Let A and D (Fig. 44) represent the two places. Place thelevel midway between… … Fig. 44. Find AA and BB\ as in the preceding case, and record theformer as a back-sight and the latter as a, fore-sight. Selectanother intermediate station C, and in like manner find theback-sight BB and the fore-sight CC; and so continue untilthe place B is reached. The difference between the sum…
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