ACOUSTICS AND GRAVITATION. 



85 



notched tubes may be placed around them, standing on the plate in question ; 

 but these are hardly needed. The beam mm' is thus easily accessible without 

 any opening of the doors of a case. The fringes occasionally move, but they 

 at once return to their equilibrium position, which is thus easily recognized. 



It seemed worth while to test this by removing the counterpoise W and pro- 

 ceeding with the method of weighing at the two ends of the beam mm', as in 

 the first part of this paper. For this reason stiff hooks, h, figure 108, were 

 fastened rigidly at each end of the beam m. The plates p, p' in the cases c, c' 

 were soldered to the bottom of h, as shown. The weight b was bell-shaped and 

 reposed on the sharp point of the hook h. A small projection on top allowed 

 of its easy removal with forceps. Adjusting the stops of the torsion-head 

 of the wire w, the bell-weight 6 could be passed (with the requisite torsion) 



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June 10 



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from one end of the beam to the other, without any inconvenience, the fringes 

 in either case of adjustment appearing at once in their proper position rela- 

 tive to the cross-wire at the slit of the collimator. Of course, h must be stout 

 enough to be practically free from flexure relative to b. 



With adequate damping a similar arrangement should also suffice for a 

 knife-edge balance, though here the plan of figure 102 would probably be more 

 available. 



Many experiments were made with the apparatus, figure 108. Naturally, 

 however smooth the results, they are ultimately all subject to the temperature 

 coefficient of torsional rigidity. 



An example of continuous results extending over 3 months is given in figure 

 109. The ordinates show the displacements of micrometer in centimeters. 

 The kinks in the curve are the results chiefly of change of temperature in its 

 effect on the rigidity of the wire. Not impossibly rapid temperature changes, 

 such as occur in the summer, may produce some yielding within the interfer- 

 ometer itself. 



