ZWARTKOPS BORE AND THERMAL SPRING. 121 



etc.. the steel easing was bottomed in hard blue clay, after which 

 the work proceeded more satisfactorily. 



It may be briefly explained here that oil wells are as a rule 

 drilled with the " jumper " variety of tools as distinguished from 

 the "diamond" drill, which is used in proving mineral fields where 

 the strata are hard, the resulting " core " being brought up solid, 

 as the drill cuts it out of the rock by means of a "crown of 

 rough diamonds " but should this diamond drill come across any 

 soft strata or clay the probability is that the tools will get clogged, 

 and result in the loss of the tools and also the hole. 



The " jumper " drill, on the other hand, is effective in either 

 hard or soft strata. Instead of beginning the hole 3 or 4 

 inches diameter, as with the diamond drill, it is begun at, say, 

 18 inches diameter, and in this case is wide enough to allow the 

 steel casing to go down as soon after the drill as possible. The 

 material, instead of coming out as a solid core, is pulverised by 

 the steel bits secured to the end of the drilling rods, water is 

 added to the stuff so pounded and the resulting sludge is brought 

 to the surface by a species of pump let down the hole by a wire 

 rope. Each bit is about 6 feet long and in breadth from 14 

 inches to ; inches, and is used according to the size of the 

 hole to be drilled. They are changed as soon as the cutting edge 

 becomes blunted, and this again depends on the hardness of the 

 rock. Usually the bit will last for two hours, but in some parts 

 of this bore a quarter of an hour was enough. The bit is secured 

 to the bottom of the rod by a screw ; there are many technical 

 and clever devices in connection with this description of plant 

 that cannot be referred to here. The work of drilling seems 

 very simple, but it requires the utmost care and experience in 

 handling the tools, as the weight runs into several tons, and a 

 small slip or carelessness may result in a bad accident, if not in 

 wrecking the hole. With the exception of a 4-foot crowbar 

 and several similar articles falling down the hole to a depth of 

 from 2.000 to 3,000 feet, there were no mishaps. These articles 

 were all recovered by improvised tools and in a short time. 

 Special fishing tools were sent out with the plant for recovering 

 heavy articles or damaged casing, but no necessity arose for 

 using them. 



The engine sent with the plant was of 35 nominal horse- 

 power, which worked an oak walking beam of great strength. 

 The outer end of this walking beam carried the string of iron 

 rods on which the bit was screwed as already described. A 

 wire rope is sometimes used for drilling in America and else- 

 where where the bore is shallow, but for deep bores, where great 

 accuracy is required, the iron rods are best. The time occupied 

 in screwing and unscrewing 3,000 feet of rods 30 feet long may 

 be considerable, but it well repays the extra trouble. 



As soon as a sufficient length of hole is drilled steel casing 

 or lining is introduced to prevent the sides collapsing. The whole 

 of this well is cased from top to bottom ; first there is about 

 So feet of 18-inch casing, inside that 060' feet of TO-inch casing. 



