22 TUNNELING 



from the two named and from each other chiefly in the methods 

 by which the valve is operated. All of these drills work either 

 with direct steam pressure or with compressed air. Workable 

 percussion drills operated by electricity are built, but so far 

 they do not seem to have been able to compete commercially 

 with the older forms. No attempt will be made here to make 

 a selection between the various forms of percussion drills for 

 tunnel work, and for the differences in construction and the 

 merits claimed for each the reader is referred to the makers of 

 these machines. All of the leading makes will give efficient 

 service. It goes almost without saying that a good percussion 

 drill should operate with little waste of pressure, and should 

 be composed of but few parts, which can be easily removed and 

 changed. 



Drill Mountings. For tunnel work the general European 

 practice is to mount power drills upon a carriage moving on 

 tracks in order that they may be easily withdrawn during 

 the firing of blasts. Connection is made with the steam or 

 compressed air pipes by means of flexible hose which can 

 easily be attached or detached as the drill advances or when it 

 is moved for repairs or during blasts. Two, four, and sometimes 

 more drills are mounted and work simultaneously on a single 

 carriage. In America it has been found that column mount- 

 ings have been more successful for tunnel work than any other 

 form. The column mounting made by the Ingersoll-Sergeant 

 Drill Co. is shown by Fig. 13. In using this form of mounting 

 no tracks or other special apparatus is required ; it is not 

 necessary, as is the case with the carriage mounting, to remove 

 the debris before resuming operations, but as soon as the blast- 

 ing has been finished and the smoke has sufficiently disap- 

 peared the column can be set up and drilling resumed. 



Rotary Drills. Rotary drilling machines, or more simply 

 rotary drills, were first used in 1857 in the Mont Cenis tunnel. 

 The advantages claimed for rotary drills in comparison with 

 percussion drills are : (1) That less power is required to drive 



