work is on a very large scale. Even in large scale maps the plumb- 

 ing arm is rarely used except on short or important sights. The 

 table should be so oriented that the survey line shall occur on the 

 paper in the general desired direction. Sight carefully at azj with 

 the edge of the ruler nearest the operator coincident with ai, and 

 draw a\ as. Also draw a short production of this line on the mar- 

 gin as indicated. With this kind of margin control, the maximum 

 permissible error of setting the base rule should not exceed three 

 minutes of arc or 4.6 ft. per mile. Without disturbing the 

 position of the planchette, sight as and a and draw the respective 

 courses accurately to scale. 



If the extension lines, like a\ and as, occur near the corner, so that 

 they may not serve as accurate subsequent guides, draw auxiliary 

 guides lines at the opposite edge of the ruler, as at a'\ a'a. This 

 plan presupposes parallelism between the edges of the base rule. 



. 12 



The Mensula Plane Table Tripod 

 \vith Ball Bearing Azimuth Adjustment 



as made exclusively by the 

 Bausrh & Lomb Optical Co. 



40 



