260 



HYDROGRAPHICAL SURVEYING [chap. xi. 



Explana- 

 tion of 

 " Dip." 



worked out, will oiily give us a portion of the height required, 

 the other portion being that below the tangent. 



Thus, in Fig. 65, A is the position of the observer, A H the 

 tangent to the earth's surface at his position, B a mountain 

 peak whose height, B D, we want to obtain. The angle of 

 elevation measured by a theodolite is B A H, and it is evident 

 that the height we shall obtain by working out the triangle 

 will be B H, leaving H D to be found independently. It will 

 be seen that we are going to treat the angle B H A as if it was 

 a right angle, when it is evidently more than 90° by the angle 



D C A at the centre of the earth ; but our figure is much 

 exaggerated to show things clearly, and in practice the dis- 

 tances we use to get elevations are so insignificant, compara- 

 tively, to the diameter of the earth, and consequently the angle 

 D C A so small, that we can neglect this quantity without 

 introducing any error in the result. With a distance of 60 

 miles, when the angle is a degree, the discrepancy introduced 

 into a height of 6,000 feet is only 2 feet. 



We require, then, to get H D to add on to B H in order to 

 get the full height of B D. This quantity, H D, is called 

 " dip," an awkward nomenclature, as it is the same used at 

 sea to express the angular quantity we apply to elevations 

 taken with a sextant from a height to reduce them to the 

 tangent to the earth, whereas here it is used to express a 

 linear quantity. 



