PHOTOGRAPHIC METHODS 



191 



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Fig. 131. 



of the apparatus depends on the action of the solenoid 

 armature. 



The Guide Rods. — If magnetic orientation fails, as, for 

 instance, with lined holes, a mechanical means must be 

 resorted to for obtaining a definite direction, 

 and his is done by means of the guide 

 rods. These are made of stiff-membered 

 cross links as in Fig. 131. In the end of 

 equal lengths of tube taps are fitted which 

 are crosswise to one another and have 

 alternate interior and exterior guide surfaces. 

 The several members are bolted in right- 

 angled planes immovable; thus the rods can 

 press against a winding borehole without 

 turning their members in shear. Over the 

 borehole the guide jack or trestle is set up 

 which gives a definite orientation to the rod 

 members as they are let into the hole and 

 for adding fresh members. Haussmann used members 75 

 cm. long, 1 cm. thick, and 4 cm. wide, strengthened above. 



The Level. — The level is used instead of a plumb bob 

 and cuts out much inconvenience; the plumb oscillates a 

 lot and slowly comes to rest and is also not so exact as the 

 level in such a narrow space. The level on the other hand 

 comes to rest quickly and its sensitiveness is quite inde- 

 pendent of the length of the plumbing cylinder and no 

 magnification of readings is necessary. 



The Crossed Magnets. — Magnetic needles are suited to 

 undisturbed regions, unlined holes, and iron-free places, 

 but one has no control over a magnetic needle. Two 

 needles close together, swinging in parallel planes, cross 

 when under contrary influences; we thus have a means 

 of locating disturbances and preventing false readings. 

 If a magnetic deflection is present the cross angle of the two 

 magnets will vary and on the vertical turning axis of the 

 magnetic needles is a differential variometer for horizontal 

 intensity. In some cases cutting out faulty orientation 

 survey spots will not give a correct notion of the survey 



