. Deep borehole surveys and problems. Boring. 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
. Deep borehole surveys and problems. Boring. 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 ordinar
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