THE ROBERTS* RANGE-FINDER. 
369 
any distance PI from I, to be read on the scale constructed as above 
by the figure p; 
then 
b _IP . _ 1600 
p~IF " P IP ' 
(vii) 
Note.—/?= 16 (hundreds understood) when IP~\, i.e. when base = 40 yds. 
Substituting the values of IF (vi) and IP (vii) in (iii) we get 
o/ _ 1600 
an equation solved by one setting on the slide rule or calculator. 
A rough slide rule, for the purpose of illustration, may be made as 
follows :—- 
Draw a scale. A, (Fig. V.) with equal divisions, and number them—» 
log 1 = 0 write 1 on scale B opposite 0 on scale A 
log 2 = ‘S „ 2 a B « 3 « A 
log 3='477 3 i, B 4-77 A 
and so on up to 10, which is written “ 1 ” on scale B, and may mean 
one followed by, or preceded by, any number of ciphers, and with the 
decimal point anywhere. All figures and readings connected with 
these scales may be similarly treated, as we are dealing with logarithms 
and can fix the mantissa of the range log by eye. 
Mark the two indices at 16 and 5055, i.e., 
Make a second scale (C) of the same total length as B, but with the 
divisions half the distance apart. 
Suppose the readings are 156 and 204 (vide Figs. II. and III). 
Set 156 on scale C under 204 on scale B; and the right-hand 
index on scale B points to 96 on scale C. One can judge by eye 
whether this means 960 or 9600 yds. (The motion is shown by the 
parallel dotted lines in Fig. Y.) 
The slide rule used is twice as long as scales B and 0, and is more 
finely subdivided than they are. The number of subdivisions on the 
slide rule between any two figures, corresponds with the number 
between the same figures on the rod-scale. 
Should the wires in the field of the tripod-telescope require adjust¬ 
ment, it may be found more convenient to alter the position of the 
t*s on the calculator to suit them, than to attempt the adjustment 
as before described. 
Place the rod at such a distance from the tripod-telescope, that the 
wires subtend the length from 16 to I. Then measure the base in 
