FIELD RANGE-FINDING. 
205 
or otherwise; (2) by wires or lines with or without marks every 5 or 
10 yds.; (3) indirectly or mechanically, by the use of a short auxiliary 
base, such as length of gun, height of man, or short tape—which latter 
may be of a constant length or otherwise; (4) by pacing, but this is 
not sufficiently accurate for long ranges. 
Examples of the foregoing are (1) Nolan's, Edwards' (class VI.); 
(2) Weldon's, constant base for Edwards' (class IV.) or sextant 
(classes III. and IV.); (3) Watkin's, Edwards' (classes I. and VII.) 
When bases are supposed to be known without measurement of any 
kind, they are generally 1 yd. long, a rigid bar or tube of metal, which 
if bent becomes unserviceable, or an axle of a carriage (Berdan), or a 
man in the enemy's ranks exactly 6 ft. high, who ought to stand on a 
glass stool for accurate observation. 
Calculation may be performed (1) mechanically, by a separate in¬ 
strument; (2) mechanically, by the same process that angles are 
measured; (3), mechanically, by the same process that bases are mea¬ 
sured ; (4) by reference to tables, ready calculated to correspond to 
each minute in angles or each foot or inch in bases; (5) by plotting 
the angles and bases to scale, and measuring* with a graduated index- 
arm, or by the intersection of two index-arms. 
Examples of the above are (1) Nolan's; (2) Watkin's, Elliott's, 
Aidie's, Poste's, Edwards' (class IV.); (3) Weldon's, Edwards' (classes 
VI. and VII.); (4) sextant (classes I., II., and III.); (5) Berdan’s, 
Edwards' (class I.) 
Having now a bird's-eye view of existing range-finding systems and 
apparatus, we may proceed to examine their adaptability to military 
purposes, as regards (1) angle-finders; (2) measurement of bases; 
and (3) calculations. 
(1) The best and most accurate angle-finder no doubt is a tra¬ 
versing telescope, provided with cross-hairs, of moderate power, but 
comparatively large field, on a steady stand. Such an instrument 
will measure an angle to an accuracy of 1 minute. But stands are 
cumbrous, and must be heavy if used in a strong* wind, and if gun 
is used, range-finding party cannot be sent in advance of battery; also, 
owing to the size of field, telescopes require vertical and horizontal 
adjustments, and the angles must be read by verniers. The next best 
angle-finders are on the principle of a sextant, which do not require 
stands, and will also read to 1 minute, but require great nicety of 
observation, unless provided with a telescope which reduces the field. 
Also there is a certain difficulty when objects are not in the same 
horizontal plane. The same remarks apply equally to prisms, except 
that the latter are chiefly used for measuring constant angles. The 
least accurate angle-finder is the traversing arm with sight-vanes, 
which will measure to within 5 or 10 minutes, but is handy to use. 
It is easy to calculate the effect of an error of one or more minutes 
on the range from the fact that the length of a circular arc whose 
radius is unity is *0002909, or 4 ft. 4 ins. in 5000 yds. 
(2) As to measurement of bases, it is exceedingly difficult to 
measure a long base accurately on broken ground, especially in a high 
