SURVEYING WITH THE TACHEOMETER. 
Ill 
a subtense instrument is not available, I found canes to be invaluable. 
They grew everywhere in the forests in Assam, and lengths of one hun¬ 
dred feet each were easily procurable. Their lightness caused them to 
float on the surface of the water, they were constant as to their length, 
and gave no trouble to the chainmen in pulling them taut in the water. 
They were also very useful in measuring through the jungle and forest 
undergrowth, through which they could be drawn without being caught 
by thorns in the bushes, advantages not possessed by chains or ropes.” 
Surveying with the Tacheometer. 
(.For description of this instrument , see p. 35.) 
The method of surveying with such a tacheometer as that shown 
(p. 36), is, as regards fixing positions of distant objects, the same 
as with the prismatic compass. This instrument has, however, this 
advantage over the prismatic compass, that distant objects are seen much 
more distinctly through the telescope, and the bearings can therefore be 
more accurately taken than when the ordinary sight vanes of the 
prismatic compass are used. In addition to which, the compass is 
larger than the prismatic compass usually carried by the traveller. The 
principal advantage of the tacheometer, however, will be found when it 
is employed for fixing positions within comparatively short distances. 
This is done by sending an assistant to the spot it is desired to fix, with a 
staff such as is shown (p. 38 or 39), and with the micrometers, measuring 
the angle it subtends when held (either horizontally or perpendicularly) 
at right angles to the line of sight, at the same time taking the compass 
reading through the prism. With the angle measured by the micro¬ 
meters, if a ten-foot staff has been used, knowing the value of the micro¬ 
meter divisions, the distance of the object can be at once obtained from 
Table XXIII. With the distance so found, and the bearing which has 
been taken, the position of the object can be at once laid down on the 
survey by setting out the bearing from the point of observation, and 
then measuring the distance, taken from the scale of the map. 
With any other length of staff than ten feet, Table XXIII. (p. 280) cannot 
be used without calculation, and the distance of the object will have to be 
computed. It is usual, when observing the angle subtended by the staff. 
