148 DEEP BOREHOLE SURVEYS AND PROBLEMS 



be altered to give any desired curvature of borehole. We 

 may get the centering motion without the worm wheel 

 gearing in other ways, e.g., by wedges displaced forward or 

 backward. Likewise in place of the electric motor other 

 power can be employed, such as valves operated by the 

 plumb or a gyroscope. The direction apparatus can be 

 fixed solid or detachable on the rods. 



The greatest demerits of the device are that it is inter- 

 mittent in action and there is no device to prevent turning 

 on insertion or extraction. 



Maillard's Apparatus. — This simple and cheap device^ 

 consists chiefly of a simple plumb-bob electrical contact 

 apparatus. Figures 88 and 89 show a longitudinal section 

 of the apparatus which is a series of hermetically sealed 

 hollow rods a connected to a body b, which has a play 

 of about 4 mm. in the hole lining A. The body h has 

 external guide springs c for centering. In the upper 

 part of 6 is a circular ebony membrane d with an opening 

 /. Below the membrane cZ is a conical recess. A cable g 

 passes through/ and holds a brass plumb bob h of cylindrical 

 shape with a spherical end. This latter rounded part of 

 the bob is the only part allowed to make contact with the 

 slanting sides of the recess; it is rounded to lessen friction 

 on being moved up or down. Cable g is an insulated 

 electric wire passing through the hollow rods a. 



It will be seen from Fig. 89 that the complete electric 

 circuit is by way of the source j at the surface, through the 

 cable g, the plumb h, the borehole casing A, the galva- 

 nometer k and back to the source j. 



When taking a measurement the apparatus is let down 

 into the hole, which is already provided with casing A, 

 by means of the hollow rods a, successively screwed up 

 at the surface in the normal way, to the desired spot to be 

 surveyed. The partial turns of the tube a which may be 

 called ai, a2 . . . an are related to a fixed starting direction 



1 Pechelbronn Societe anonyme d'exploitations minieres et Georges 

 Maillard. French Patent July 27, 1925. German Patent No. 492,573, 

 Mar. 4, 1930. 



