MISCELLANEOUS WORK 197 



In the above sketch, the first base hne is from B 

 toD. 



Poles are set up in line between these points, the dis- 

 tance measured, and found to be 500 yards. 



A cross-staff is then used to find out where a straight 

 line from A would meet BD at right angles. These lines 

 are called offsets. The commonest form of cross-staff 

 consists of a block of wood having in it two saw-cuts 

 exactly at right angles to each other, fixed to a 

 pole about 5 feet long. The latter should be pointed 

 and shod with iron to make it easy to stick in the 

 ground. 



This instrument is set up on line BD, where it is judged 

 that the line from A will meet it at right angles. A sight 

 should be taken through a saw-cut, getting it in Hne with 

 B and D, then a sight taken through the other saw-cut. 

 If this does not point straight to the offset staff erected at 

 A, the cross-staff must be moved to right or left as re- 

 quired until it does. 



A special instrument called an " optical square " is 

 made. With it a man walking along line BD and looking 

 at D through the instrument would see the offset staff 

 at A reflected in a small mirror when he arrived at the 

 spot where the hne should cut BD. 



The mirror is set at an angle of 45 degrees with the 

 main line of sight, so that any object reflected in it is 

 exactly at right angles with the line of sight. 



When the correct spot, J, is found and marked, the 

 line J A is measured — viz., 200 yards. The cross-staff is 



