142

BOOK V.

measurement become a greater length than it should be, he sets upon theinstrument a standing plummet level, the tongue of which, if the instrumentis level, indicates no numbers, but the point from which the numbers start.

i\ i

Compass. A, B, C, D, E, F, G are the seven waxed circles.

When the surveyor has carefully observed each separate angle of thetunnel and has measured such parts as he ought to measure, then he laysthem out in the same way on the surveyor’s field 20 in the open air, and againno less carefully observes each separate angle and measures them. First ofall, to each angle, according as the calculation of his triangle and his artrequire it, he lays out a straight cord as a line. Then he stretches a cord at

a0 It must be understood that instead of “ plotting ” a survey on a reduced scale onpaper, as modern surveyors do, the whole survey was reproduced in full scale on the“ surveyor’s field.”