510 REPORT—1890. 
wu, and B the chief observer’s instrument. The scale s, attached to B, 
slides relatively to a mark m,, which may be supported upon a rifle or 
light staff or simply held in the hand. The assistant observer adjusts his 
position with reference to this mark and the distant object, while the 
chief observer moves his instrament and scale forwards or backwards till 
he sees the assistant observer’s mark in contact with the distant object. 
He then reads his scale by reference to an index attached to the piece 
M,. This instrument may also be adjusted once for all, and so con- 
structed as to be incapable of being put out of adjustment. 
Fig, 15. Fria. 16. 
Q 
The distance between two distant and inaccessible objects can readily 
be determined by any of these telemeters, but most conveniently perhaps 
by the first one. For this purpose the scale is set to read any convenient 
multiple of the base, the base-cord is discarded, and the telemeter used as 
a fixed-angle telemeter in a manner similar to that described with refer- 
ence to fig. 2. For example, let the scale for 25 yards base be set to 
read 1,000 yards, giving a multiplier of 40. One of the observers takes 
up a fixed position 8, fig. 16, and sights one of the distant objects o!. 
The other observer moves out in the direction BA! set out by B’s instru- 
ment (guided by directions of ‘forward’ or ‘ back’ from B) till he sees 
B’s mark reflected upon the object 0', while B sees A’s mark also reflected 
on 0}. The distance Ba! will then be ~, of Bo!. The observer A 
leaves a mark at A', and then proceeds in like manner to find the point 
4? corresponding to 0%, B remaining at the same point. Then Ba? will 
be 5 of Bo, and, since the angle a! 8B a? will evidently be equal to 
0' Bo’, the distance a! a? will be 31, of the distance 0! 0? between the two 
distant objects ; in fact, the triangle a! 8a? will be a map of the triangle 
