370 
THE ROBERTS* RANGE-FINDER. 
yards and, having squared it, use the figures thus obtained in place of 
the left-hand t (10)- The two t*s are always half the total length of 
the scale B (from 1 to 1 on scale C) apart, so the second t is easily 
obtained. It may also be found by dividing the first by \/To. 
When the two figures 1 ** on scale B and C coincide, the scale C 
reads off the squares of the scale B, so that there is no difficulty in 
finding the square or square root of any number. 
The reason for the above correction of the t is,— 
Equation (ii) may be written (IT) 2 =cot 2 0 x (P/) 2 , where 6 is the angle subtended 
by the wires. 
but as^? is made equal to 1600 
(/T) 3 =cot 2 0; and cot 2 0 is the true reading for the left index on scale f>. 
