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ON SOME NEW TELEMETERS, OR RANGE-FINDERS. 499 
On some New Telemeters, or Range-finders. By Professors ARCHI- 
BALD Barr, D.Sc., M.Inst.C.£., and Witu1aM Stroup, B.A., 
D.Sc. 
[Ordered by the General Committee to be printed in extenso.] 
Tue conditions of modern warfare have given rise to the requirement of 
efficient range-finders for artillery and infantry use and for coast defence. 
Such instruments are, however, not only indispensable for military 
purposes, but civil engineers have long felt the want of reliable telemeters, 
both for use in rapid survey work of a more or less rough and preliminary 
kind, and for making accurate measurements under conditions not 
favourable to the application of ordinary methods of surveying. 
With a view of meeting these requirements a great variety of 
instruments have been devised, a few of which have been brought into 
use, and have proved more or less successful in practice. For coast 
defence may be instanced the position-finder of Major Watkin, in use in 
England, and that of Lieutenant Fiske, used in America; for artillery and 
infantry purposes the instruments of Watkin, Weldon, and Labbez; and 
among instruments for surveying purposes the stadiometer, tacheometer, 
and omnimeter. 
Instruments have been devised for military range-finding to indicate 
the distance of an enemy by a measurement of the time-interval between 
seeing the flash or smoke from one of his guns and hearing the report; 
but such a method of operation could not, for obvious reasons, serve all 
the purposes of military range-finding, and could not be relied upon for 
purposes either of attack or defence. Setting these aside, we may say 
that all range-finders and telemeters—properly so called—depend for 
their indications upon the measurement of the elements of a triangle, one 
of whose sides is the range to be determined. In nearly every case, 
too, the triangle to be solved is approximately right-angled, and the 
operation of determining the range of a point o from 4 (fig. 1) consists 
virtually in setting out the base aB, and measuring the angle subtended 
by it at 0, or in setting ont the angle at 0 and measuring AB. In such 
cases, as the side 4B—which is referred to as the base of operation—is 
very small compared with the range 04 (or 0B), the distance of 0 from 
A (or B) will be expressed, with sufficient accuracy, by AB/a, where 
a=angle A OB in radian measure. 
Telemeters naturally divide themselves into two classes —(1) those 
using a base of known or observed length at the distant object ; (2) those 
working from a base of known or measured length at the observer's 
station. Instruments of the former class, using as base the distance 
between two marks (or the interval between two observed graduations) 
upon a staff heid at the distant point (e.g., the tacheometer and omni- 
meter), are, as a rule, the most accurate and convenient for surveying 
work when the distant point is accessible to a man carrying the base- 
staff. We hope on a future occasion to describe some new telemeters of 
this ciass which we have recently devised. Proposals have been made 
to determine the distance of an enemy by means of simple instruments 
working upon this principle, and using the height of a man in the 
enemy’s ranks as a base; but besides the impossibility of getting reliable 
results, under the most favourable circumstances, with a base so ill- 
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