172 
MINUTES OF PROCEEDINGS OF 
All these instruments agree in having for base their own length, and in 
superposing, rendering identical, or measuring the distance between two 
images of the observed object which enter at either end of the telemeter, and 
which are brought together at the centre by prisms, mirrors, or lenses. 
These telemeters are generally a yard long, as it would be difficult to use 
longer ones in the field. 
These instruments may become useful auxiliaries in action, but they can 
never be sufficient alone for artillery purposes— 
(]) Because they require exquisite fineness of observation. 
(2) On account of the extreme minuteness of the divisions on the scale. 
(3) From their liability to being thrown out of adjustment, and the difficulty 
in detecting the error. 
1st. If instead of exactly superimposing the two images, an error of an inch 
is made, there will be an error at 1800 yds. of 50 yds. 
2nd. To give good results, the scale must be divided to seconds; and any 
one who has used either a sextant or a theodolite knows how difficult it is to 
read hurriedly to seconds. 
3rd. The third is, however, the fatal drawback ; it is quite 
evident that the slightest flexure in the base must ruin the results, 
not by sensibly affecting the length of the base, but by com¬ 
pletely altering the set of the optical ends of the telemeter. In 
addition to this, the prisms, lenses, or mirrors of all instruments are 
liable to slight alterations of position or shape. 
These arise from the unequal bearing of internal strains, and from 
changes of temperature ; all human workmanship is liable to derange¬ 
ment from these two causes, but the effect is intensified in this class of 
instruments from the smallness of the base, and from the liability 
of the base to change its shape. 
Some reliance could, however, be placed on these instruments 
were it possible to detect their error, and to allow T for it. But 
this I consider impossible, unless another class of instrument is also 
carried. 
Colonel Clerk proposes to test his instrument by measuring 100 
yards by a cord, and testing on this known distance. 
This remedy is fallacious, because if the 100 yards is measured 
only 9 inches wrong, the telemeter will, at 3,000 yards, be 225 
yards wrong. 
Class 3.—Instruments which use a fixed base. 
Nearly all these use bases of 100 yards, and employ one instrument 
to fix a right angle, and another which measures a second 
angle. 
Colonel Clerk, however, used two angle measurers (sextants) in 
a system distinct from that noticed in Class 2. 
The objection to a fixed base is insurmountable, as ground—par¬ 
ticularly the little hills for which guns instinctively make in 
combat—varies too much in shape. It has been endeavoured to 
overcome this difficulty by making out tables, but that interferes with 
the requisite simplicity ; certainly, if the tables are made out for every 
