M, 



Photographic methods 183 



by a spring g attached to the bottom screwed plug 

 /i.i A series of screws i in the side of the tube in 

 straight parallel lines project inward about 3^f e in. 

 There are slots down the cylindrical cases carrying 

 the lamps, and the needle and bob, the screws i 

 acting as guides for these slots. 



A dry battery k and clock j with spiral spring I 

 attached are in the top half of the tube. This 

 spring I assures contact when the two halves are 

 screwed together by pressing on the top end piece 

 m. This latter has a ball-bearing swivel n for 

 attaching the wire for lowering if needed. 



The vulcanite cases p carry the brass marine 

 compass attachments for the magnetic needle. 

 The plumb-bob attachment is also vulcanite for 

 insulation purposes. The compass swings in rings. 

 On the face of each gimbal is a fixed pin point and 

 round the edge is a recessed ring which holds the 

 disc of sensitized photographic paper in place, the 

 pin points holding them in position. The plumb 

 bob is made of gold attached to a fine silk thread 

 swung from the center of a thin disc of plate glass 

 q, which fits into a recess in the tip of the vul- 

 canite case. 



Both the magnetic needle and the plumb bob 

 swing immediately above (almost touching) the 

 sensitized papers. A copper lug s attached to the 

 extra wheel r of the watch makes, at a certain 

 time, connection with a copper spring t attached 

 to the frame of the watch and so completes an 

 electric circuit lighting the lamps above the bob and 

 needle. Now a sharp shadow of each is photo- 

 graphed on the sensitized paper; these are devel- 

 oped to give dip and direction by making the pin 

 pricks coincide, as shown in Fig. 121, where the 

 hole dips 25 deg. in a direction N.20°E. (magnetic). 



^ ® V & y Pjq J20. 



1 Hoffmann, J. I., Recent Practice in Diamond Drilling and {After Hoff- 

 Borehole Surveying, Trans. Inst. Min. and Met., April, 1912. mann.) 



