142 BOOK V. 



measurement become a greater length than it should be, he sets upon the 

 instrument a standing plummet level, the tongue of which, if the instrument 

 is level, indicates no numbers, but the point from which the numbers start. 



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



When the surveyor has carefully observed each separate angle of the 



tunnel and has measured such parts as he ought to measure, then he lays 



them out in the same way on the surveyor's field^" in the open air, and again 



no less carefuUy observes each separate angle and measures them. First of 



all, to each angle, according as the calculation of his triangle and his art 



require it, he lays out a straight cord as a line. Then he stretches a cord at 



^"11 must be understood that instead of " plotting " a survey on a reduced scale on 

 paper, as modern surveyors do, the whole survey was reproduced in full scale on the 

 " surveyor's field." 



