G. H. HALLIGAN. LXXVII. 



or other material is scoured away, the rate of progress depending, 

 of course, on the material to be passed through. 



The highest boiler pressure available to the author is 110 lbs. 

 to the square inch, and this has seldom been required. Any- 

 suitable pump may, of course, be used, but a 6 x 4 x 6 Worthington 

 steam pump has beea found in every way satisfactory. When a 

 band of material harder than the water jet will readily overcome, 

 is met with, a drill is lowered to ascertain its nature. If it 

 should be clay an auger is sent down and the material augered 

 out till the sides of the hole begin to show signs of falling in. 

 The tubes are then driven to the depth reached by the auger 

 and the process repeated till a softer or harder material is met 

 with. The pump is, of course, kept going to soften as much as 

 possible the clay or mud, and to keep the material already passed 

 through from settling round the tube. The clay auger has a 

 pitch of 4 in., and this has been found to work best for clay 

 similar to that overlying the Hawkesbury sandstone. For small 

 gravel or shells a shell auger is used, and for large gravel or drift 

 a double corkscrew auger is the best. 



When rock is met with it is drilled to a diameter of 3^ in., 

 and under-reamed to allow the tubes to be lowered. The sludge 

 is withdrawn by the sand-pump (shown on the diagram). The 

 hinged-valved sand-pump is too liable to be clogged with grains 

 of sand or small gravel to be reliable, so the ball-valve pump is 

 now invariably used. 



An entirely satisfactory under-reamer has yet to be designed 

 The best tool for under-reaming by percussion is that known as 

 Ball's Under-reamer, or rather a modification of it, embodying 

 two improvements. In the original Ball Under-reamer it was 

 necessary to insert a plug of soft wood at A (Fig. 2) when the jaws 

 closed ready for lowering into the tube. When the appliance 

 reached the bottom this plug was supposed to be thrown out by 

 the contraction of the spiral spring, thus allowing the jaws to 

 open to the full extent (as shown on the diagram). This seemed 

 feasible enough, but the plug did not always answer to the call. 

 It either fell sideways and thus allowed the jaws to open to only 



