214 Proceedings of the Royal Society of Victoria. 



Appendix A. 

 On the design of Pendulum Apparatus for Differential Observa- 

 tions of Gravity. By E. F. J. Love, M. A. 



The work of the Gravity Survey Committee, of which I have 

 the honour to be secretary, has forced upon my attention the 

 subject of the various theoretical points which come up for 

 consideration in the designing of pendulums for use in differential 

 observations of gravity. A good many different forms of 

 pendulum are now employed for this purpose by observers in 

 different parts of the world : and it seems not altogether without 

 interest to examine the various types in use, with the view of 

 seeing to what extent they agree with what we may look upon as 

 the ideal pendulum, and, where they differ, to indicate the 

 manner in which improvements might be effected. 



(a) The first point to which attention should be directed is 

 the fact that the only measurement which has to be made on 

 pendulums for differential work is the determination of the 

 vibration number in different parts of the earth. This at once 

 places the possible accuracy of the work on an altogether 

 different level from that obtainable in any absolute measurement 

 of gravity : in absolute measurement we are definitely limited by 

 the degree of accuracy obtained in the measurement of the length 

 of the pendulum, an operation very inferior in this respect to the 

 determination of a vibration number by the method of conici- 

 dences. But the whole value of the work depends on the extent 

 to which the vibration number of the differential pendulum at 

 any one place maintains its constancy ; in other words, on the 

 degree of invariability of the pendiilum. 



It might seem superfluous to insist on the necessity of so 

 constructing a differential pendulum that it shall be, as far as the 

 nature of material structures will allow, invariable in form and 

 dimensions, but such is not the case. Some observers have 

 conceived the idea that it would be convenient to be able to 

 make both differential and absolute measurements with the same 

 pendulum : notably Commandant Defforges,* who has con- 



* Comptes-Rendus des seances de la Commission rermanente de I'Associatiou Gtodesigne 

 Internationale, 1888. Annexe vb. p. 12, eij<j. 



