WELLS AND WELL SINKING. 



Holes 



system is not suitable for very hard rocks, as the ex- 

 cessive driving required will damage the 

 joints. The method which was described 

 as the Chinese system is known as per- 

 cussive boring, and it was stated that the 

 action of raising and lowering the tools 

 imparted the necessary rotary motion. 

 So far as it goes, the system is useful for 

 small bore-holes started at the surface, 

 but for deeper and longer ones, and those 

 which are started from the bottom of a 

 well, as a continuation thereof, when the 

 work has to be done from a staging as low 

 down in the well as possible (which is 

 usually a few feet above the supposed 

 level to which water will rise), iron rods FlG - 3 1 - 

 take the place of the rope. The staging is a plank 

 flooring securely cross-braced, having a hole in the 

 centre for the rods to pass through, but it must be 

 small enough to prevent the passage of a special clamp 

 attached to the top rod in use, which will form a rest 

 for the apparatus while changing rods and guard 

 against loss of the tools below the stage. Below this 

 opening, and down to the bottom of the well, a 

 temporary iron tube is placed to act as guide. The 

 boring rods may be from 10 to 20 ft. long, and are 

 generally made of wrought iron and sometimes of steel. 

 The top rod is of the form shown by fig. 32, and has 

 eye-holes for the turning bars (called " Tommy " bars) 

 to pass through a hook, by means of which the drop 

 motion is imparted. The rod joints are made like 

 the one in fig. 33, in which the recess is shown for 

 use with the clamp referred to above. This clamp 



