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FIELD RANGE-FINDING. 
Under these circumstances—- 
I. Base and base angles may vary. (This is the surveyor's method, 
and best for surveyors.) 
II. Base and one base angle may vary, the other base angle being 
constant. 
III. Base may be constant, the base angles varying. 
IY. Base and one base angle may be constant (generally a right 
angle), the other base angle varying. 
Y. Base may be constant, the base angles varying but equal. 
YI. Base may vary, the base angles being constant. 
YII. Base may vary, the base angles constant and equal. 
The following may be mentioned as examples of these seven classes :— 
I. (1) The surveyor's plan—viz., theodolite and chained base; (2) 
the military surveyor's or reconnoitring plan—viz., pocket 
sextant or prismatic compass, and base measured by chain 
or by pacing; (3) Nolan's service range-finder; and (4) 
Edwards' range-finding plane table. 
II. Watkin's field range-finder. 
III. (1) Berdan range-finder (German) ; (2) pocket sextant, in con¬ 
junction with 50 or 100 yds. base and tables. 
IY. Adie's telemeter, containing its own base 1 yd. long; (2) Captain 
Poste's micrometer; (3) Edwards' range-finding sextant; (4) 
pocket sextant, in conjunction with 50 or 100 yds. base, and 
tables or “ scales of tangents " for each minute difference in 
the variable angle from 85° to 90° or from 80° to 90°. 
Y. (1) Elliott's micrometer telescope, depending upon a man 6 ft. 
high or height of a mounted man as base; (2) some of the 
yard telemeters. 
YI. Edwards' (broke down at Aldershot on unfavourable ground, 
but remarkably rapid and accurate on the flat). 
YII. (1) Weldon's; (2) Edwards'. 
Space will not admit of describing and considering the merits or 
demerits of each of the above in detail, but it may be observed that 
classes I. to Y. are intricate systems, classes YI. and YII. are simple. 
All are practicable under favourable conditions, and all are reliable 
except the yard telemeters in classes IY. and Y. 
Angles are principally measured (1) by traversing telescopes (with 
or without cross-hairs) on tripod stands or other support (like a theo¬ 
dolite) ; (2) by reflecting 1 mirrors held in the hand (like sextants, 
optical squares, &c.); (3) by totally reflecting prisms, which with the 
preceding mirror arrangements are used in conjunction with telescopes 
or otherwise; (4) by a traversing arm provided with sight-vanes on 
tripod stand. 
Specimens of the above are (1) Nolan's, Berdan; (2) pocket sextant, 
Watkin's, Weldon's, Edwards' (classes IY., YI., and YII.), some of the 
yard telemeters, &c.; (3) Weldon's, Aidie's, Edwards' (class YII.); 
(4) Edwards' (class I.) 
Bases are measured (1) by tapes graduated in yards, feet ; and inches. 
