PRACTICAL FORESTRY. 



b g, c l y and d n, are inserted in a column headed Foresights, 

 and the heights a e, b h, c Jc, and d n in a collateral column 

 headed Backsights. The difference between the sums of 

 the numbers in these two columns will be equal to the 

 height of one extremity (a) of the line, above the other (d). 

 When a number of levels have to be taken in succession it 

 will be found a saving of time to use the Surveyor's level or 

 theodolite instead of the ordinary spirit level illustrated 

 on previous page. 



With regard to the use of the instrument as shown in 

 Fig. 1. it may be necessary to state that the height of the 

 eyesight b from the ground must be deducted from the point 

 observed. As an example : if the object-pole or staff be 

 marked in feet and inches, and the hair wire in taking a 

 sight strike the same at, say, 8 ft., then, if the eyesight be 

 4ft. from the ground, the difference of level between the two 

 stations (instrument and station) will be 4 ft., that is, there 

 will be 4 ft. of a fall from the spirit-level station to that of 

 the station where the staff was placed. On the other hand, 

 should the hair wire strike the object pole at, say, 2 ft. from 

 the ground, chese 2 ft. must be deducted from the height of 

 the eyesight, which, as we have said, was 4 ft., then the 

 ground at the station-pole must be 2 ft. higher than that at 

 the spirit-level. 



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