CHAP. xi.T LEVELLING 269 



There is no necessity to keep in one line directly for the 

 spot whose height we wish to measure ; we shall do so if we 

 can, as it is the shortest way, but in practice we are generally 

 forced to zigzag. 



The difference of the sums of the readings of back and fore 

 stations will be the height required. Where former are the 

 greatest, we are going up hill, and it is called rise ; vice versa 

 is called a fall. 



The following is another example of entries in the Field 

 Book in connecting two bench marks, A and B. 



0-589 



0011 



0-005 

 0-125 



0-057 



+ 0-141 0-646- 



0^141 + 



Diflference of Level .. .. .. 0-505 — 



The levelling staves should not be too far from the theodo- 

 lite ; about 50 yards is quite far enough with a 5-inch 

 theodolite and large telescope, if accurate readings are 

 required. 



The theodolite should be, as far as possible, at equal distances 

 from both staves, in order to avoid having to alter the focus 

 of the telescope. 



As far as possible, place the theodolite in the line joining 

 the two staves, with two opposite levelling screws in the line 

 of the staves. 



