COMPASS AND PLUMB-BOB METHODS 137 



The device is certainly simple and ready for use at any 

 time without delay or special preparation, and it is suited 

 to manipulation by the ordinary boring personnel. Of its 

 drawbacks we may mention: 



1. The pressing down of the plate; the needle and pendu- 

 lum point might be injured by this but it might be regulated 

 by controlling the piston pressure. 



2. The waste of time brought about by letting hollow 

 rods into the hole. 



3. It has no special centering device and it must be 

 exactly centered. At great depths unbalanced rod loads 

 upset instruments not specially centered. 



Dr. Freise of Aachen speaks of the apparatus being 

 shrouded in trade secrecy, a feature that unfortunately does 

 not apply to this apparatus alone. 



Gallacher's Apparatus. — This apparatus was invented in 

 Johannesburg in 1918 and possesses features of remarkable 

 ingenuity and mechanical skill. It is designed to survey 

 both the deviation from the vertical and from the azimuth, 

 these to be read direct from the instrument on withdrawal, 

 without surface calculations. We enter into some detail 

 respecting it here because it appears to have lacked the 

 necessary publicity such a device merits. 



It consists of an outer casing a (Fig. 3, Plate VIII) with 

 an inner casing b longitudinally pivoted in it by means of an 

 adjustable footstep bearing in the bottom end c. 



The inner casing (Figs. 1 to 2a, Plate VIII) carries the 

 controlling and recording elements in the form of clock- 

 work, plumb bob (in the shape of a weighted cylinder) and 

 compass, and spring device for controlling the clockwork 

 and fixing the plumb bob and compass at any desired spot. 

 This inner casing is suitably cut away and windowed oppo- 

 site its weighted side. 



Figure 1 shows the inner casing at the clockwork d end; 

 Fig. la shows the other end of the inner casing carrying the 

 cylindrical plumb bob e, compass / and the clamping device 

 g for both. 



