THE TacHeoMeTeH 
39 
frame e is screwed to the bar, and the other end is fixed by a thumbscrew 
c in such a position that the line joining b a is at right angles to the line 
joining the discs. For travelling, the thumbscrew c is unscrewed and 
the frame is closed up against the bar, in which position the thumbscrew 
screws into the hole d in a metal plate affixed to the bar. The bar is 
fixed in posit on by an assistant looking along the sights a , b, and laying 
them on to the theodolite. 
Fig. 3 represents another form of rod and one more easily made, 
though not calculated to give such accurate results. A A are two 
boards, one foot square, painted white, with a black cross on each. 
These are fastened on a bamboo, B B, in such a manner that the centres 
of the crosses shall be a known distance apart. 
When using the rod in a vertical position it will often be found con¬ 
venient to fasten a stick to it, so that it shall extend about two feet beyond 
one of the boards. This, when placed on the ground, takes the weight of 
the rod and helps the assistant to keep it steady. 
Any theodolite can be used as a tacheometer, by having hairs in the 
diaphragm fixed at such a distance apart as to read one foot on a staff 
when it is one hundred feet distant from the instrument, two feet when 
the staff is two hundred feet distant, and so on, and a theodolite fitted 
in this manner will always give a proportion of 1 to 100 between the 
reading on the graduated staff and the distance. As the power of the 
telescope is usually small,, the figures and marks on the graduated staff 
can only be read at a comparatively short distance. 
The following precautions must be taken, or no accurate results can be 
obtained. The fixed hairs must be adjusted to read in the proportion of 
1 to 100, or, what is the same thing, the staff must be marked to read one 
foot, when it is 100 feet distant from a certain point. It is the determina¬ 
tion of where this point is that is absolutely necessary, and the place from 
which to measure the distance is arrived at in the following manner:—■ 
