442 



BORING 



being again lowered, connected to the rods and drawn as above until the whole of the 

 rods are drawn out. There is a brake attached to the drum on the main shaft, which 

 is used for lowering the rods into the hole. 



In cases where the strata passed through are of a soft character, tubes are 

 sometimes driven forward as the hole progresses, these at first being about 4J inches 

 diameter and faced with steel. They are driven forward by a large iron weight, 

 and should the first pipe meet with anything that it cannot drive through, smaller 

 sized tubes are introduced. 



The chisel used is about th of an inch smaller than the lining tubes. 

 This machine has bored in chalk a depth of 76 feet in 12 hours. 

 The Diamond Boring Machine. In all the modes of boring described above, the 

 j n borer or drill acts by percussive impact against the face 



of the strata, producing a volume of powdered rock equal 

 to the solid contents of the borehole. In the diamond 

 machine, invented by M. Leschot, the cutting tool is of a 

 tubular form and receives a uniform rotating motion ; 

 the result being the production of a cylindrical core from 

 the rock of the same size as the inner periphery of the 

 tube, showing to great depths the different stratification 

 through which the boring tube passes. The general prin- 

 ciples on which the machine is constructed are as follow : 

 A continuously revolving and progressing boring head, 

 having projecting diamond points, works in combination 

 with a tubular boring bar. 



The boring bit (fig. 179, is a steel thimble, about 4 inches 

 in length, having two rows of Brazilian black diamonds 

 (such as are used by lapidaries) in their natural rough state 

 firmly imbedded therein, the edges projecting slightly from the outer and inner peri- 

 pheries respectively. The diamonds in the centre row cut the path of the drill in 

 its forward progress, while those Tipon the outer and inner periphery of the tool 

 enlarge the cavity around the same and admit the free egress and ingress of the 



