THE WORLD '8 ADVANCE 



187 



HIGH SPEED REPLACES WEIGHT 

 IN DRILLING ROCK 



Quite a contrast in methods of ex- 

 cavating rock is furnished by the con- 

 struction of the new subway system in 

 New York as compared with the first 

 New York subway, which was com- 

 pleted in 1904. In the former case prac- 

 tically all the rock was drilled by means 

 of heavy reciprocating drills mounted on 



total weight of from 500 to 1,000 pounds 

 for the mounted type. The strange fea- 

 ture about these little drills is that, in 

 spite of their size, they make faster prog- 

 ress than the heavier type. The secret 

 of their great cutting speed is due to a 

 number of factors. In the first place 

 they strike three or four times as many 

 blows per minute. The cutting steel 

 does not reciprocate, but rests against 

 the rock while a rapid succession of 



New and 

 Old Meth- 

 o d s of 

 Drilling 

 Rock for 

 Blasting. 



heavy iron tripods. Weight was the pre- 

 dominating factor the apparatus was 

 heavy and the blow was heavy, but the 

 speed of operation was correspondingly 

 slow ; in fact, the apparatus was so cum- 

 bersome that it frequently required as 

 much time to set up the drill as to drill 

 the blast hole. 



Hand drills without any mounting 

 whatsoever are the principal means 

 adopted for cutting the rock in the new 

 subway. These little drills, known as 

 Jackhammers, represent the other ex- 

 treme. They weigh from forty to ninety 

 pounds, depending on size, as against a 



hammer blows strikes its upper end. At 

 the same time the drill automatically 

 rotates the steel and blows a jet of air 

 down the hollow steel so as to eject the 

 cuttings and constantly present clean 

 rock to the cutting edges. 



Furthermore, the dead time formerly 

 consumed in setting up and adjusting is 

 entirely eliminated w r ith the new type of 

 machine, as it requires but an instant 

 to apply the drill to any spot desired, 

 and the drill can be used conveniently 

 in all sorts of out-of-the-way locations 

 where cutting by any other method 

 would be extremely difficult to accom- 



