WELLS AND WELL SINKING. 



35 



to that of the shaft, less twice the thickness of con- 

 crete and 4 in. = 2x6 + 4=16 in., and is made in 

 two parts joined by iron plates C, C, and bolts. The 

 frame can also be made collapsible by cutting away 

 a taper piece of the ring at D, D, between the plates, 

 and the four boards which form the outer circum- 



FIG. 



FIG. 14. 



ference of the frame are left loose. The rings are 

 made by drawing a circle the size of the frame laying 

 the boards round its circumference (see fig. 16). The 

 boards are then lightly tacked together and a circle 

 of the same radius marked on three ends. After the 

 boards are knocked apart they are sawn out along 



