WELLS AND WELL SINKING. 43 



one. The sliding bar is attached to the rope, and 

 consequently when the tool descends the top teeth 

 will engage with the top collar, while when it is raised 

 the bottom teeth engage with the bottom collar, and 

 the rope will twist through an angle corresponding 

 to the distance of half the pitch at each reversal of 

 the motion. 



Yet another form of rock-boring is that in which a 

 diamond drill is employed, and which may be used 

 for bore-holes up to 2 ft. in diameter. It has the ad- 

 vantage of cutting a solid core, which is both easy of 

 removal in large sections and also serves to show 

 the engineer the exact nature of the strata cut 

 through. The motion is purely rotary, and the 

 cutting is done by diamonds securely brazed into 

 steel plugs, fitting into an apparatus called the crown, 

 which revolves according to the hardness of the 

 rock from 50 to 200 revolutions per minute. The 

 advantage of rotary drilling is, of course, the solid 

 cores produced, besides the reduced liability to 

 breakdowns and failure ; because with percussion 

 boring in fairly large holes, and to great depths, the 

 moving parts of the apparatus become very heavy, 

 and vibration sets up in the rods which is difficult 

 to prevent. This originally led to the adoption of 

 wooden rods, iron-bound and with iron heads and 

 feet, screwed with male and female threads in the 

 usual way. Wood rods, however, are liable to break, 

 due to the twisting action required ; and to remedy 

 this the hollow steel rod was introduced, which 

 appears to have given fairly good results. With 

 great depths, however, even with the use of tubular 

 or wooden rods, the boring tool is always in danger 



