24 



Anniversary Meeting. 



[Nov. 30, 



Having learnt that a series of pendulum experiments at the principal 

 stations of the Great Russian Arc were in contemplation, I availed myself 

 of an opportunity of informing M. Savitsch, by whom the operations were 

 to be conducted, that the Invariable Pendulums which had been employed 

 in the English experiments were now in the possession of the Royal Society, 

 and, being unemployed, would, I was persuaded, be most readily lent by 

 the Society on an application to that effect being made. The constants of 

 these instruments, including the coefficient in the reduction to a vacuum, 

 having been most carefully determined, they were ready, with the clocks 

 and stands belonging to them, for immediate use, and would have the 

 further advantage, that experiments made with them in Russia would be 

 at once brought into direct connexion with the British series extending 

 from 79° 50' N. to 62° 56' S. latitude. The communication was most 

 courteously received and replied to. It appeared, however, that a detached 

 invariable pendulum had been already ordered by the Russian Government 

 from M. Repsold, of Hamburg, shorter than the English pendulums for 

 convenience in land transport, and with two knife-edges and two fixed 

 lenses, symmetrical in size and shape but one light and the other heavy, 

 and so arranged that the times of vibration should be the same on either 

 knife-edge in air of the same temperature and density. M. Savitsch ex- 

 pressed his desire to bring this pendulum in the first instance to Kew, 

 and to secure thereby the connexion of his own with the English series, 

 where also he would have the opportunity of testing the exactness of the 

 correction for buoyancy by vibrating his pendulum on both its knife-edges 

 in the vacuum-apparatus which is now established at Kew. 



It is much to be desired that a similar series of pendulum experiments 

 to those about to be undertaken in Russia should be made at the principal 

 points of the Great Indian Arc ; and the steps which are understood to 

 be in progress in providimg new instruments for the verification of the 

 astronomical and geodesical operations of the Trigonometrical Survey of 

 India, and to give them a still greater extension, would seem to present a 

 most favourable opportunity^ for the combination of pendulum experiments. 

 In such case the pendulums of the Royal Society might be made available 

 with excellent effect. 



The large size of our printed volumes in the present year gives no 

 unfavourable and, I think, no unfair idea of the present scientific activity 

 of the Society ; for I believe it may be safely said that our Council has not 

 been less vigilant and cautious than heretofore in the selection of the papers 

 to be printed. Although much care has been given to keeping the ex- 

 penses of illustration within reasonable boimds, the cost of the Society's 

 publications has been this year unusually high ; yet I am glad to be able 

 to state that our whole expenditure within the year has fallen within our 

 income. With your permission, I will briefly advert to a few of the sub- 

 jects which have occupied the Society's attention in the past year. 



