270 HYDROGRAPHICAL SURVEYING [chap. xi. 



correc- For our purposes, when the distance of the staff from the 



^hen theodolite is not great, or when the distances of fore and back 

 necessary, station from the theodohte are nearly the same, it will be 

 sufficient to observe readings with the telescope in one posi- 

 tion only : but when the rise of the hill is slight and distances 

 increase, especially when the difference of distance between 

 fore and back station is great, and we require accuracy, the 

 telescope should be reversed in the Y' s, and being again brought 

 level by the bubble, readings shoilld be taken a second time. 

 The mean will be the true reading. 



If the axis of the telescope and the attached level are per- 

 fectly parallel, and therefore in adjustment, it ^vill be shown 

 by the readings agreeing when reversed at the first station, 

 and we shall know that we need not take this trouble ; but 

 it is necessary to ascertain this, as theodolites continually 

 undergoing carriage by boat are liable to many accidents. 



This method enables us, if necessary, to calculate the height 

 of any of the stations where the pole is erected, but gives us 

 no information as to the height of the spots where the theodo- 

 lite stands. This can be obtained, if wished, by measuring 

 the height of the axis of the telescope above the ground, w^hen, 



Height of theodolite position = height of back station 

 + present reading of said back station — height of eye 

 (back station being below us). 



Distances measured will enable us to make a section of 

 the ground traversed, but, as already remarked, this is not 

 often required from the marine surveyor, and Avill not be 

 enlarged upon here. 



