500 REPORT—1890. 
defined and so variable in length, it is seldom that observations of the 
kind required can be made at all under modern systems of warfare. The 
only range-finders suitable for general military purposes, therefore, belong 
to the second class. 
Telemeters which utilise a base at the observer’s station may, again, 
be divided into two sub-classes—(a) instruments having a short rigid 
base, and usually arranged to be operated by one observer, such as those 
of Adie, Christie, Mallock, and Haskett-Smith, and one recently brought 
out by the present writers; (b) instruments working from long bases, 
say 20 to 50 yards, and requiring two observers (or one observer 
observing successively from the two ends of the base), such as Weldon’s, 
Lynam’s, and Watkin’s artillery and infantry range-finders. The three 
instruments which we are about to describe also belong to this division, 
to which we shall alone refer in what follows. 
Fi4. 1. Fig. 2. 
oO 
0 
g 
>» 
iS 
© 
S 
4 8B (eS Es Pe tt Sa 
A<-Measured---rB 
116 devs ' 
These long-base instruments may yet again be subdivided into two 
groups—(a) those using constant angles and variable base, such as the 
Weldon; and (f) those with constant base, one constant angle, and one 
variable angle, such as Watkin’s and Lynam’s infantry range-finders. 
Major Watkin’s artillery range-finder practically belongs to this class as 
well, though it admits of the base being arbitrarily chosen within certain 
limits. 
Colonel Weldon’s instrument consists essentially of two triangular 
doubly-reflecting prisms, ground to give the angles at the base of a 
right-angled triangle of which the base is 3'5 of the perpendicular 
(which is the range required). The mode of operation of this range- 
finder is illustrated in fig. 2. It is usually worked by one observer, who 
stations himself at A, and observes with the 90° prism what distant object 
at ¢ appears reflected into coincidence with the object 0. He then walks 
out along the line cA produced to B, where c appears in coincidence with 
o when using the second prism. The distance aB is then measured by a 
cord or tape marked at every two yards to represent hundreds of yards of 
range, and subdivided to indicate tens of yards. 
Instruments of the constant-base and variable-angle type have 
