220 Proceedings of the Royal Society of Victoria. 



uniformity of temperature throughout the enclosuro, and should 

 be painted of a light colour externally, so as to diminish the rate 

 of absorption of heat : it cannot very well be left bright, as the 

 light reflected from it would be a serious obstacle to observing. 

 The apparatus should of course be rigid and tolerably heavy, 

 otherwise the pendulum when swinging will set it in vibration; it 

 should also be a form of very stable equilibrium, else external 

 disturbance may shake it and interefere with the motion of the 

 pendulum. A truncated cone of thick brass seems to answer 

 every purpose ; glass windows can be introduced where necessary, 

 and the case should be mounted on large levelling screws, which 

 stand in metal grooves on a heavy support of stone or timber. 

 Such an apparatus need not weigh moi'e than half-a-hundred- 

 weight or so all told, and need take up but little room ; a striking 

 contrast to the Indian apparatus, which is extremely bulky and 

 weighs nearly a ton. 



Appendix B. 



Fart of a Letter from Sir G. G. Stokes, Bart., P.R.S., to the 



Secretary. 



7 Queen's Parade, Bath, 



6M August, 1891. 

 Dear Mr. Love, 



You do not say expressly, but I take for granted that in the 

 contemplated gravity survey you mean to use invariable pendu- 

 lums, not Kater's pendulum, or son)e other form available for 

 absolute determinations. It is generally, I think, allowed that 

 for determining the variation of gravity from place to place the 

 results obtained by invariable pendulums are the more accurate. 

 The series of determinations would be rendered absolute by 

 transporting the pendulums to sorne station where gi\avity has 

 been well determined absolutely and swinging them there. It 

 will suffice if the station last inentioned be one for which gravity 

 is accurately known absolutely by comparison, by means of 

 invariable pendulums used by previous observers, with some 

 other station where gravity had been determined absolutely. 



