( 83 ) 
Hints on Use oe Sextant in Surveying. 
{For the description of this instrument , see p. 15.) 
To measure the Angular Distance between two Terrestrial Objects . 
When the horizontal angles between terrestrial objects have to be taken 
with the sextant, the index is set to zero (0°), and the instrument must 
be held in the right hand in such a manner that its plane is parallel to 
an imaginary line joining the two objects; put back all the dark shades, 
and, looking through the telescope collar and the horizon glass at the 
right hand object, unclamp the index and move it slowly forward until 
the reflected image in the mirror of the horizon glass coincides with the 
other object seen directly; clamp the index and make the coincidence 
perfect with the tangent screw, then read the angle. Make it a rule to 
commence taking the angles from the object farthest to the right, then 
from the next farthest, and so on, always working from right to left. 
By so doing mistakes will often be prevented in plotting the work, and 
you will be able to recognise the objects from which angles have been 
measured in your rough sketch. Avoid very large or very small angles, 
as they may cause considerable errors in the positions assigned. Should 
it be required to measure the horizontal angle between two objects, one 
of which is at a considerable elevation above the other, as a tree on a plain 
and a mark on the top of a hill, it will be necessary to select some object 
immediately below the mark on the hill, and as nearly as possible on the 
same level as the tree, and measure the angle subtended by them. If no 
object in a suitable position can be seen, select some point about 90° or 
100° from one of the objects, and observe the angles between each object 
and that point; the difference between these two angles will be the 
horizontal angle, nearly. Should the angle be too large to be taken in one 
measurement, the object to the right must be brought by reflection ta 
some well-defined mark, and the reading taken; the angle must then be 
measured between the mark and the other object; the sum of these 
g 2 
