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MINUTES OE PROCEEDINGS OP 
100 yds. base, at other times they will, apparently, unaccountably fall off: 
the explanation being that the address required for very accurate work 
cannot be always maintained. 
2ndly—When telescopic power is adapted to sextants they become 
clumsy. 
3rdly—The same man cannot observe from both ends of the base; this 
last objection is of no consequence when good distinguishable points, such 
as a steeple, a target, or an isolated man are observed, but it is fatal when 
the points are bad or confused, as they will probably be in the field. 
In addition to the three grand classes of instruments discussed above, 
other systems have been proposed, such as finding ranges by the focussing 
of a telescope, or by similar triangles; I have not entered upon these 
because they are all thoroughly unworkable. 
Nor have I touched on the instruments which are used from 
heights. 
To sum up, I do not see how any instrument can possibly be con¬ 
structed to equal the range-finder, unless it adopts the principle on which 
the range-finder is founded, and the artifices which have enabled that 
principle to be applied to artillery purposes. 
The principles which I have adopted arose from the following idea 
having occurred to me. 
I observed that all surveyors’ plans for finding distances succeeded, 
and that they invariably employed three variable quantities—the base 
and the two base angles; but that, on the other hand, all military plans 
for finding ranges depended on, at most, the variation of two quantities, 
and nearly all upon the variation of a single quantity, and that all 
military plans had failed. The range-finder is the surveyors’ method 
adapted for use in action and by gunners. 
The same result might have been attained by using sextants, but I 
consider them unsuitable for artillery purposes, for the reasons above 
given. In thus rejecting them, I do not depart from my main idea, 
as all writers on surveying agree that the sextant is not a convenient 
instrument with which to take the angles of terrestrial objects. 
Theory. 
I had not originally intended to proffer a theoretical explanation of the 
range-finder, because it can be correctly worked without any knowledge 
of mathematics, and because the very name of theory might, in some 
minds, create prejudice against the instrument. I, however, annex 
the following account of the principles upon which the range-finder 
is based, in compliance with a request of Colonel Gordon, C.B., when Chief 
Instructor of Gunnery, Slioeburyness. 
The experience of ages has shown surveyors that the most convenient 
way of measuring distances is to measure a variable base, and two variable 
base angles. They scarcely ever follow any other method, such as using 
a fixed base, or one fixed angle. 
I imitate the surveyors’ proceeding, but add these conditions, that the 
base must be Short, and each of the base angles greater than 80°, and less 
than 100°. 
