and 

 hence, 



HEIGHT MEASURING. 21 



M C = D C x tan. m ; N G = D C x tan. n 

 M N = I = D C (tan. m + tan. n} 



DC = 



tan. m + tan. n 



By introducing this value into the former equation, the 

 height is obtained as : 



TT _ I x (tan. a + tan. 6) 

 tan. m + tan. n 



N 



Fig. 13. 



All instruments which measure vertical angles are suited 

 for trigonometrical height measuring. For practical purposes 

 it is desirable that the instrument should not require a stand, 

 and that, besides the angles, the corresponding tangents should 

 be marked on it. 



c. Description of some of the more useful Instruments. 



Weises Instrument. It consists of (1) a tube (T) with an 

 objective in the shape of a cross at one end (0) and an eye- 



