THE TRANSIT 
79 
must cut off the unit distance on the rod not at a distance 
of 100 or of 66 feet, but at a greater distance equal to the 
distance from the center of the instrument to the objective 
lens + the distance from the cross-wires to the same lens 
when focused on a distant object. This correction, (/ + c) 
as it is called, is about 1 foot in common transits. 
In testing the instrument on measured bases, therefore, 
these should be measured out from the plumb line or 
center of instrument to the required distance + the 
constant above described, and for accurate determina¬ 
tion of distance the constant should be added to the 
distance observed. In working out inclined sights from 
the table this constant may be added to the rod reading 
before the reductions for horizontal distance and elevation 
are made. 
In the practice of woodsmen, however, work will generally 
be accurate enough if this constant is. neglected, all the 
more so since this error tends to be compensated by that 
irising from neglect of the small vertical angles noted above. 
There are, indeed, a few transits so constructed that no 
such constant correction as that above stated has to be 
considered. 
Accuracy. The accuracy of stadia measurement de¬ 
pends largely on the state of the atmosphere. If that is 
hazy, or unsteady from the effects of heat, long shots can¬ 
not be taken and measurements on shorter distances 
cannot be accurately obtained. There is furthermore the 
nossibility that the line of sight by the lower hair when 
oassing over very hot ground may be refracted more than 
:he other and thereby give too small a reading. Other¬ 
wise than here and above stated the only sources of in¬ 
accuracy are due to errors in rod readings which for small 
errors are as apt to be + as — and so mainly balance one 
another. Thus while on single shots stadia measurement 
pay be appreciably inaccurate, the relative error decreases 
with the length of the line run. 
In general it may be said that stadia measurement gives 
satisfactory results for very many purposes, and that it has 
great advantages in the way of rapidity and cheapness. 
With good instruments and clear air it can be employed 
