22G Mr R. Threlfall, On an automatic mechanical [Mar. 2, 



March 2, 1885. 

 Prof. Foster, President, in the Chair. 



The 'following communications were made to the Society: — 



(1) On some theorems in Tides and Long-waves. By Rev. E. 

 Hill, M.A. 



Elementary considerations were given from which it might be 

 inferred that when a disturbing body produces a semi-diurnal tide 

 in an equatorial canal, the point nearest to the disturbing body 

 will be a point of low tide or high tide according to the depth of 

 the canal. 



A general explanation was given of the influence of the depth 

 of a canal on the speed of a long- wave traversing it. 



It was shewn that the ordinary formula for this speed might 

 be deduced from the ordinary differential equation of motion 

 without integration. 



(2) On the electrical resistance of Platinum at high temperatures. 

 By W. N. Shaw, M.A. 



(3) On an automatic mechanical arrangement for maintaining 

 a constant high potential. By R. Threlfall, B.A. 



In connection with certain experiments which I wished to 

 undertake some time ago it became desirable to obtain an appara- 

 tus capable of maintaining a condenser at a high potential for 

 a considerable period. Although further consideration led me to 

 abandon the experiments referred to, pending an enquiry into 

 certain preliminary questions, I am of opinion that the arrange- 

 ment I devised at the time for maintaining a high potential, may 

 be of some independent interest, and this the more since I believe 

 that there are many experiments which would be greatly facilitated 

 if there was a ready method of maintaining condensers at a 

 constant mean potential during considerable periods. Where the 

 leakage is very small, as in Sir William Thomson's electrometers, 

 much satisfaction may doubtless be obtained by the hand use of 

 the replenishing guage at short intervals of time. There are cases 

 however in which the fall of potential may be very much more 

 rapid than in these instruments ; and it is such cases that the 

 arrangement to be described has been arranged to meet. 



In the Proceedings of the R. Society for 1881 Lord Rayleigh 

 has shewn how a water motor of the Thirlmere type may be 

 arranged to run at an approximately constant speed ; viz. by 



