PENDULUM METHODS 165 



line will be shown. A graduated sight on the uppermost 

 link is used for orientating in the vertical against the coor- 

 dinate axes. Thus the plumb line can be viewed at any time 

 and a new marking plate can also be put in at any time. 

 In this way any doubtful measurements can be recognized 

 at once and remedied at any time, an advantage which did 

 not hold for the predecessors of this apparatus. In previous 

 instruments a series of measurements below each other 

 necessitated separate readings and extractions for each, or 

 separate depth readings at each place with all the attendant 

 trouble and waste of time. Again errors increase with the 

 depth. 



This apparatus can be arranged in lengths to suit the hole. 

 For a 240-m. hole, say, Thurmann would not employ sixty 

 4-m. tubes but ten or at most twenty tubes respectively 

 24 or 12 m. long. There is a special plumb for each section 

 of hole surveyed so that any errors cannot be cumulative. 

 Moreover, each error can be corrected, as said above. 

 Therefore it is only necessary to correctly orientate the 

 whole apparatus from the surface down, and to aid this 

 direction rods (Fig. 9) are used. These are a series of tubes 

 equal in length to the link tubes and having tooth and 

 notched ends connected by overscrewed thimble joints 

 to prevent them rotating. The above noted diopter is 

 adjusted to the direction rods on exactly the same line 

 as is chosen for the uppermost plumbing section of the link 

 tube. In this way the coordinate axes of the marking 

 planes lie sectionally in exactly uniform orientation for 

 plumbing. 



Freezing shafts are best plumbed from the center by this 

 device, the center being the coordinate axes center. 



The inventor claimed that the method was cheaper than 

 its predecessors for freezing shafts and also surer; that it 

 was unaffected by water, mud, chemicals or pressures and 

 that it was direct and easily controlled. Among its 

 demerits we may mention: 



1. There is insufficient provision against relative turning 

 of the tubes; this spoils the deduced results. 



