104 MINIIJG INDUSTEY. 



The outer timbers of" each set, that is, the two sides and ends of the n:iain 

 frame, are 14 inches square ; the posts, 10 in number, four at the corners and 

 two at each end of the three partitions, are of the same size. The dividing 

 timbers, forming the partitions, are 12 inches square. These partitions, as 

 may be seen in the drawing, are not close, no planking or lining being used on 

 them, but two pieces of timber are employed, one above the other, at each set, 

 leaving open spaces of about 3 feet between the sets. The drawing on Plate 

 III presents an isometric view of a series of these sets, forming together the 

 timbering of the shaft. The lagging or sheathing is removed from the side 

 and end nearest the observer. 



The ground through which these shafts pass being generally too unsettled 

 to allow of sinking to any considerable depth without support, the work of 

 timbering is necessarily done from above downward, as the sinking pro- 

 gresses. The method of putting these timbers in place is about as follows : 

 When sufficient ground has been excavated below the last set of timbers, for 

 conveniently putting in another lower set, the long horizontal timbers, or sill- 

 pieces, forming the sides of the set, already framed for receiving the ends, and 

 having gains cut for the posts and cross-pieces, or ties, are lowered down and 

 put approximately in place, being hung by chains to the last set already fixed 

 above. The sill-pieces are usually in two parts, each about 13 feet long, 

 butted together, at the middle, without splice or framing. The ends, cross- 

 pieces, and posts are then fitted as nearly as possible into their proper places. 

 This being done several long, round iron bolts, each made in two parts with a 

 tightening screw in the middle, are passed through the new set and the one, 

 or sometimes two or more, above. Everything being approximately in its 

 place, the new set is adjusted exactly to its proper position, by means of the 

 tightening screws on the bolts by which it now hangs to the set above. The 

 lagging is then put in behind the timbers, and between the plank and the 

 ground are inserted pieces of spiling and wedge-timber, which are driven into 

 place or forced in by jackscrews as firmly as possible. Once fixed in this 

 manner, everything is held by lateral pressure ; the bolts by which the set 

 was at first suspended are allowed to remain for a time and then withdrawn, 

 for use in placing succeeding sets. 



The shafts on the Comstock timbered in this manner generally stand 



