264 HYDROGRAPHICAL SURVEYING [chap. xi. 



To get this distance, if we happen to have it worked out 

 in the tri angulation, we shall of course use the calculated 

 distance ; but if not — which will be the case generally — we 

 must measure it on our sheet, and enter the corresponding 

 distance according to the scale. 



In the column headed Corr", enter the correction for dip 

 and refraction, obtained as above by multiplying the distance 

 by 100 and dividing by 4. 



We then work out the difference of height with these data 

 on the opposite side of the page, the constant log being the 

 log of feet in a mile, by which the distance must be multiphed 

 to bring result out in feet. The log given is that for 6,075 feet, 

 the number of feet in a mile in Lat. 44°. Theoretically, we 

 should have a different log for different latitudes, but, as the 

 utmost extent of error by neglect of tliis is 22 feet in a height 

 of 6,000 feet, we need not regard it. This difference of height 

 is entered in its proper column. 



Tables, computed by Commander Purey Gust, for taking 

 out the difference of height for any angle and distance, are 

 now supplied. If these are at hand, tliis computation is 

 dispensed with. 



The column for height of theodolite is a little confusing, 

 as sometimes it will be merely the height of the theodolite- 

 telescope above the ground, and sometimes the height of it 

 above the sea-level, which we shall enter, according as we 

 want the height of observer's position or of object observed, 

 as will be presently explained. 



We have now all the data necessary to obtain heights. 



When we have accumulated enough observations, we set 

 about getting out results. 

 Height There are four problems for obtaining heights, and the data 



Problems. ^^^ have for each observation will be combined according to 

 what we want to arrive at. 



These four problems are as follows : 



1. To find height of object observed, when height of 



observer is known, and the angle is one of elevation. 



2. Ditto, when angle is one of depression. 



3. To find height of observer, when height of object 



observed is known, and the angle is one of elevation. 



4. Ditto, w hen angle is one of depression. 



