Course of Civil Engineering, Vol. 2: Comprising Levelling, and the Construction of Common Roads, Railways, Canals, Harbours, Docks, Tunnels, ... Piers, Water-Works, &C (Classic Reprint)

Excerpt from Course of Civil Engineering, Vol. 2: Comprising Levelling, and the Construction of Common Roads, Railways, Canals, Harbours, Docks, Tunnels, Aqueducts, Viaducts, Piers, Water-Works, &C

Levelling is a delicate and important branch of engineering. It enables us to find the exact difference of level between any number of places, and to represent the contour of any portion of the country through which it may be necessary to run a road or canal. A well connected series of levels points out the tions to conduct water, either for th drainage or irrigation. In conducting an extensive trigonometrical survey, the difference of level between stations is found by means of reciprocal angles of eleva tion and depression taken with a theodolite, which, in such operations, answers the purpose very well. In surveying, the theodolite being fixed at every station, no time is lost in preparatory adjustments to take the vertical angles, subtended by remote stations, whose dis tances are known by calculation. The vertical angles and distances being known, and due allowance made for the effect of the earth's convexity, and the influence of refraction, a series of levels may be taken very ex peditiously, and sufficiently accurate: but for expedi tion and accuracy it is far inferior to the levelling instruments now in use. The theodolite, therefore, is never used for levelling purposes, except in the case of a trigonometrical survey.

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