PHOTOGRAPHIC METHODS 



185 



Many successful borehole surveys have been made with 

 this apparatus and W. Gallacher, the inventor of the 

 instrument illustrated in Plate VIII, added to the above 

 ancillary devices various means for obtaining successive 



y. 



I 



Fig. 123. Fig. 124. 



Fig. 122.— PUot wedge. 



Fig. 123. — Payne-Gallwey's wedge surveying attachment. 

 Fig. 124. — Guide wedge. 



deflections in the same hole. It was also a wedge device. 

 Mr. Hoffmann^ gives several instances of its successful 

 application. 



1 Op. cit., p. 9. 



