COMPASS AND PLUMB-BOB METHODS 151 



It would be difficult to imagine a simpler device and it has 

 recently been protected in Germany. We may visualize 

 the following possible defects: 



1. The angular positions ai, a2, etc., being dependent on 

 the inner rods are not free from objections. 



2. Friction of the cable at the membrane and hindrance 

 to the same should pressure water and mud penetrate the 

 many joints. 



3. The device may become cumbersome in deep holes. 



4. The borehole must be lined all the way. 



The Driftmeter. — This is a recently developed American 

 apparatus^ being a pricking plumb-bob device. The instru- 

 ment (Fig. 91) is about 3>^ ft. long and weighs about 30 

 lb. and is suited for rope lowering with a depth-measuring 

 appliance or it may be fitted to the rods. The principal 

 parts are the clock, the ten 13-^-volt batteries, the leaden 

 plumb bob fixed on a solenoid or electromagnet and the 

 magnet-controlled pricker plunger passing 

 through a universal bearing which has a 

 mobile suspension. Under the pricker is 

 a 2%-in. registering paper (Fig. 92) divided 

 into 15 circles of 1 deg. each and is thus 

 suitable for filing. Space is provided on the 

 back of this paper disc to record depth, well ^^^- 92.— Drift- 



mctGr rGcord 



number and other data. In this way deflec- 

 tion angles are found direct to about 15 min., no preliminary 

 work being necessary, the instrument being ready for use 

 as soon as a new paper disc is fitted and the clockwork set. 

 The clock can be adjusted to a definite time; then by the 

 contact brush making connection with the battery and 

 magnet the plunger is set into action perforating the paper 

 disc. A retracting spring keeps the plunger off the paper 

 when the current is shut off. The resulting reading is 

 direct and needs no computation. The same sheet can be 

 repeatedly used by marking each perforation as made, so 

 getting a series of indications of the deviation. Since it 

 requires no special skill the ordinary boring personnel can 



1 The Driftmeter Co., Inc., Tulsa, Oklahoma. 



