On the Secular Change in the Magnetic Dip in London. 223 



to 95° C. in two hours, and maintained at the latter temperature 

 during four hours more. Diffusion appeared to be accelerated 

 about six times at the higher temperature. 



At low temperatures, again, diffusion is proportionally slow. 

 The ratio of diffusibilityof the following salts at two different tem- 

 peratures appeared to be, — 



For chloride of potassium at 5°*3 (41°*5 F.), 1 ; at 16°*6 (62°*0 F.), 1*4413 



For chloride of sodium at 5°-3 (41°-5 F.), 1 ; at 17°*4 (63°*4 F.), 1*4232 



For nitrate of soda at 5°-3 (41°*5 F.), 1 ; at 17°*4 (63°*4 F.), 1*4475 



For nitrate of silver at 5°*3 (41°-5 F.), 1 ; at 17°*4 (63°*4 F.), 1-3914. 



The salts are unequally affected to a sensible extent ; and it 

 will be observed that the superiority of chloride of potassium over 

 chloride of sodium, in diffusibility, is increased at the low tem- 

 perature. 



Within the range of temperature of the preceding experiments, 

 the diffusibility of chloride of sodium being taken as 1 at 17°*4 

 (63°-4 F.), it becomes 0*7026 at 5°*3 (41°*5 F.) ; or it diminishes 

 0*0246, or ~, for a depression of 1° (0*0136, or -~ b) for a de- 

 pression of 1° F.). 



[To be continued.] 



XXXII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from vol. xxii. p. 552.] 

 March 14, 1861. — Major- General Sabine, R.iV., Vice-President and 



Treasurer, in the Chair. 

 npHE following communications were read : — 

 -*- " On an Application of the Theorv of Scalar and Clinant Radical 

 Loci." By Alexander J. Ellis, Esq.,"B.A., F.C.P.S. 



"A Seventh Memoir on Quantics." By Arthur Cayley, Esq., 

 F.R.S. 



" On the Secular Change in the Magnetic Dip in London, between 

 the years 1821 and 1860." By Major- General Edward Sabine, R.A., 

 Treas. and V.P.R.S. 



I propose in this communication to bring together and discuss four 

 determinations at different epochs, in which I have myself been either 

 directly or indirectly concerned, which have had expressly in view the 

 object which forms the title of the paper. . 



Epoch o/1821. — The experiments on this occasion were made in 

 a part of the Regent's Park, then occupied as the nursery garden of 

 Mr. Jenkins : an unexceptionable locality in all respects, and far 

 distant at that time from buildings or iron implements, railing, or 

 pipes. The experiments, ten in number, were made on six different 

 days, between the 3rd and 10th of August 1821 ; and all between 

 8 a.m. and 4 p.m. The circle employed was 11| inches in diameter, 

 made by Nairne, a celebrated artist in his day for instruments of this 

 description : the needle was made by Dollond on Professor Tobias 



