RANGE-FINDERS. 
211 
which, with the tape, weighing a few ounces, forms a complete set of 
apparatus; so there is not much to complain of on the. score of 
portability. 
The range-finding party should consist of three men, to attain 
maximum speed, and the following are the rules for guidance in using 
the apparatus. 
The range-finding party being at A ( (Fig. 1), and the range of an 
object 0 required, No. 1 and No. 2 agree upon a prominent part of it, 
and make a rough guess at the range (an even number of hundreds 
of yards), and divide by 20, which gives the length of the main base 
approximately. Supposing the range to be estimated at 2400 yds., 
then 120 yds. will be about the length of the main base. Both No. 1 
and No. 2 now direct their telescopes on the, object, and observe what 
object on the horizon (or otherwise) at D is seen by reflection at or 
near 0. No. 2 runs out 120 yds. in the direction AD, No. 1 watching 
him, and calling* “ Bight (or left) incline /' 3 if he diverges from the 
line AD. On completing 120 yds. No. 2 turns about, and directs his 
telescope on No. 1, while No. 1, looking at No. 2 through his telescope, 
moves slightly right or left until he sees the object O exactly coin¬ 
ciding with No. 2 by reflection. No. 2 observes whether the reflection 
of the object 0 falls to the right or the left of No. 1. If to the right, 
he retires; if to the left, he advances straight towards No. 1. When¬ 
ever No. 2 moves, No. 1 maintains the true angle at A by moving 
slightly right or left, which is only necessary when No. 2 does not 
move perfectly straight. When No. 2 arrives at the point B, where 
the object and No. 1 coincide by reflection (No. 1 being also correctly 
placed at A), both No. 1 and No. 2 stand fast. This is the first part 
of the process, and is much the same as Major Weldon's. The second 
operation is to find and measure AC, the third proportional to the 
range OA and the main base AB. No. 1 makes a half-turn, faces the 
object O, and lines No. 8 on it; No. 3 taking the ring-end of the tape, 
and No. 1 allowing the tape to run out as far as the graduation marked 
2400 (the estimated range), where he holds it. No. 3 stretches the 
tape taut, and looks at No. 1 through his telescope, moving “in'” 
(towards No. 2) or “out” (from No. 2) as directed by No. 1, who 
places No. 3 so that when he looks at .the centre of No. 3's instrument 
he sees No. 2 coinciding therewith by reflection. No. 3, looking at 
the centre of No. Ns instrument, sees the reflection of No. 2 either 
right or left of it. If left, he retires; if right, he advances. No. 1, 
who must not move at all, corrects No. 3's position after each move¬ 
ment, by saying “In” or “Out.” When No. 3 arrives at C, the 
true position, the distance between the centres of the instruments, AC, 
is measured with the tape, and No. 1 reads the range at once by 
inspection. 
Though lengthy to describe, the whole operation of finding a range 
only takes 1^ minutes. 
It will be observed that No. 3 ; s movements are the converse of 
those of No. 2. ITence the following general rule :— 
When working on the right of a base (whether principal or 
auxiliary) retire if the reflection of the distant object falls to the right 
