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VII. OH Qimrtz Thread Gravity 



By RICHARD THRELFALL, lati-fi/ Professor of Physics in tlie University of Sydney, 

 and JAMES ARTHUR POLLOCK, Demonstrator of Physics in the. University of 

 Sydney. 



Communicated by Professor J. J. THOMSON, F.R.S. 



Received, April 11, Read, April 27, 1899. 

 [PLATES 13, 14.] 



THE problem of replacing dynamical by statical methods of studying the variations of 

 the earth's gravitational force has long occupied the attention of astronomers and 

 physicists, and a good many attempts have been made to construct tin instrument 

 which should enable relative measurements of gravitational force to lie carried out 

 with a smaller expenditure of time and trouble than is incidental to the observation 

 of pendulums. 



The only kind of force which is practicable as a means of opposing gravitation in 

 the construction of such an instrument as is here contemplated, is that derived from 

 the elastic properties of matter. 



Accordingly the problem is reduced to that of constructing either a spring balance 

 of sufficient accuracy, or of making use of the elastic properties of a gas. 



In both cases, when we approach the limits of accuracy obtainable by jH-ndulnm 

 observations, we have to face very great difficulties arising from the necessity of ascer- 

 taining the temperature of the apparatus within very narrow limits, and this obviously 

 implies the even greater difficulty of insuring that the apparatus shall have the same 

 temperature at every point. 



If we add to this the consideration that the apparatus must lie reasonably portable, 

 and of such construction that it is not possible to disturb its mechanism l>v the shaking 

 inseparable from transport, it is evident that we have to face a mechanical and physical 

 problem of considerable difficulty. 



Until Mr. BOYS discovered the unique properties of fused quartz, in 1887,* no 

 material having elastic properties of the requisite simplicity and constancy was avail- 

 aide, ami from this cause, if from no other, all attempts at constructing a statical 

 instrument of reasonable accuracy must necessarily have failed as they all did. But 

 even setting this aside, we are satisfied that all the designs some of them of great 



* 'Phil. Mag.,' June, 1887, p. 489. 



S.I 2.99. 



