78 FIELD GEOLOGY. 



direction of the tree, we place our assistant at the mile- 

 stone with the staff on the B. M., and walk on to where 

 a rise in the ground begins. In doing so we may 

 observe whether the chain-man is directing his leader 

 properly along the line he should do this by looking 

 along the chain and by a wave of the hand to either 

 side, getting it ranged properly before the arrow is stuck 

 in the ground. Having set up the instrument and 

 booked the length, ten chains, in the level-book, we 

 send on the chain-men to measure to the next fence. 

 Adjustment for horizontality having been made, we 

 direct the -telescope back to the head of the staff on the 

 B, M., clamp it, and by turning the tangent-screw get 

 the cross-wire exactly to coincide therewith. We read 

 off the result as a depression of '50', and enter it in 

 its proper column, making a little cross also in the 

 column headed "Back-sight" to indicate that this is 

 such, take another look through the telescope and at the 

 reading to see that there is no mistake, and beckon the 

 assistant to come on. He is sent on immediately to 

 stand in the fence to which the chain-men have mea- 

 sured, in such a position that he can be seen both from 

 the instrument and from the ground beyond. Another 

 sight is now taken on the staff and entered, this time 

 as a " fore-sight " again checked by repetition, and we 

 walk on to find that the measurement from the instru- 

 ment to the fence was 4 chains 33 links. We go on 

 again with the chain- men, set up the instrument, take 

 a back-sight, and, in fact, repeat exactly the former 

 operations. As stations for the instrument and for the 

 staff, the spots on the line where changes occur in the 

 shape of the ground should be selected, the smaller 



