BAR-SUBTENSE SURVEY. 
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angles to the line of sight until he is covered by the second fixed hair, 
when he might be stopped by some pre-arranged signal, and place another 
staff there. He must then carefully measure the distance between 
these two staves, which distance multiplied by the ratio between the 
value of the hairs, which is generally 1 in 100, will be the distance of a 
point, midway between the two staves, set up by the assistant, and the 
observer. Thus, if the measured distance between the staves was 10 
yards, the distance from the instrument would be 10 x 100 - 1000 yards. 
Surveying on the tacheometer principle, but without a tacheometer, 
may be carried to greater distances in the following manner. 
Supposed a densely wooded plain over which it has been impossible 
to preserve any record of the distance travelled, but with elevated 
country at its extremities, the distance between points on the elevated 
lands may be very accurately found by measuring a base on one at 
right angles to the position on the second, of such a length that it will 
subtend an angle of two or three degrees to an observer at the second 
point; and marking these ends either by choosing conspicuous trees or 
other marks, or by flashing from them with a mirror, or by making fires. 
The observer obtains the angle by a sextant or theodolite between the 
ends of the base, and by simple right-angled trigonometry calculates the 
distance. 
Bab-Subtense Sukvey. 
At the meeting of the British Association at Cardiff, 1891, the late 
Colonel H. C. B. Tanner, Indian Staff Corps, read a paper on Bar-Subtense 
Survey, from which the following is extracted:— 
The Bar-Subtense method has none of the drawbacks attending the 
use of the chain or of micrometer instruments; it is more accurate than 
either, and is effected by means of an ordinary theodolite, together with 
bars of varying lengths, according to the nature of the work to be 
performed. 
The system is readily acquired by native surveyors after a week’s 
instruction, and in their hands, over the roughest possible mountain 
tracts, is capable of furnishing horizontal measurements up to a maximum 
of some two miles with an error of about three feet per mile, and up 
VOL. I. I 
