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 

 arising 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 

 possibility that the line of sight by the lower hair when 

 passing over very hot ground may be refracted more than 

 the 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 

 may 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 



