CHAPTER XI 



HEIGHTS 



By Theodolite — By Sextant — Obtaining Distance from Elevation of a known 

 Height — Levelling. 



Foe, obtaining heights we must mainly depend on angles of Means 

 elevation with sextant from afloat, and of elevation and ^^^^' 

 depression with theodolite from shore stations. The pocket 

 aneroid, though useful, as described under " Topography," to 

 get subsidiary heights and assist in delineation of hills, is not 

 to be depended upon. 



At all main stations, and, in fact, any station \\ ell fixed stations 

 and conveniently placed, angles of elevation and depression 1°^^°^" 

 to the objects whose heights we want, should be taken tlii'ough- Heighus. 

 out the course of the work. These are entered into the 

 Height Book, and worked out when we can get the dis- 

 tances and occasion offers, the results being tabulated and 

 meaned. 



Elevations and depressions can be taken from any station 

 whose height we shall eventually know ; but it is evident that 

 any slight error in the true height of the observing station will 

 be carried on into all heights deduced from it, and therefore 

 it is well to get as many observations as we can from stations 

 at the water-level, or so placed that the height above the 

 water-level can be measured with a line. 



In observing elevations and depressions with a theodolite, Use of 

 (he instrument must be in fair adjustment, and carefully j^J'®°'^°' 

 levelled, and it is further necessary to take into account the 

 errors of level and collimation. 



There are two ways of doing this. One is to take a series of 

 observations with the telescope in its ordinary position, and 

 then another with the telescope reversed, end for end, in the 



257 17 



