It is evident that the lower sign alone suits our case, for d < r; consequently,
(9)
Having obtained C, we put the instrument in the direction A B C. Then each point of C F describes a circumference of the same center o.
16. If the distance of the points A and B were too great, then it would be easy to determine a series of points belonging to the arc of circumference sought (Fig. 4).
Being given C, the direction C I, and C I = R, on C I I lay off C E = d, draw A E B perpendicularly, and calculate C A or A E. I shall have
or, as absolute value,
(10)