Royal Society. ^^ 



sphere, the result was found to agree ahnost exactly with Bessel's 

 experiments on the time of vibration of a sphere swung in water. 



As a limiting case of the problem of a ball pendulum, the author 

 has deduced the resistance of a fluid to a sphere moving uniformly 

 under such circumstances that the square of the velocity may be neg- 

 lected. The resistance thus determined proves to be proportional, 

 not to the surface, but to the radius of the sphere ; and therefore 

 the quotient of the resistance divided by the mass increases very ra- 

 pidly as the radius decreases. Accordingly, the terminal velocity of 

 a minute globule of water descending through the air depends almost 

 wholly on the internal friction of air. Since the index of friction is 

 known from Rally's pendulum experiments, the terminal velocity can 

 be calculated numerically for a glol)ule of given diameter. The ve- 

 locity thus calculated proves to be so small, in the case of globules 

 such as those of which we may conceive the clouds to be formed, 

 that the suspension of the clouds does not seem to offer any diffi- 

 culty. Had the pressure been strictly equal in all directions in air in 

 the state of motion, the terminal velocity of such globules would 

 have been far larger, and consequently the quantity of water which 

 could have existed in the air in the state of cloud would have been im- 

 mensely diminished. It appears therefore that these small and hi- 

 therto almost unrecognized forces, which depend on internal friction, 

 are essential to the fertility of at least the tropical regions of the 

 earth. 



The author has also applied the theory of internal friction to the 

 calculation of the subsidence of a series of oscillatory waves. On 

 substituting for the index of friction in the resulting formula the nu- 

 merical value deduced from the experiments of Coulomb, it appears 

 that in the long swell of the ocean the effect of friction is insignifi- 

 cant, whereas in the case of the short ripples excited on a small pool 

 by a puff of wind the subsidence due to friction is very rapid. Accord- 

 ingly, short ripples of this kind quickly die away when the breeze 

 that excited them ceases to blow. 



KOYAL SOCIETY. 

 [Continued from p. 256.] 



Jan. 16, 1851 Magnetic Survey of the Eastern Archipelago. 



Bv Captain C. M. Elliot of the Madras Engineers. Communi- 

 cated by Lieut.-Coi. Sabine, V.P. Treas. R.S. &c. Received Jan. 15, 



In' the vear 1845, the Committee of Physics of the Royal Society 

 having expressed a wish that a .Magnetic Survey should be made 

 of the East Indian Archipelago, Captain Elliot was ordered by the 

 Court of Directors of tlie East India Company to undertake that 

 duty, after the close of the Singapore Magnetic Oljservatory. 1 he 

 observations at that observatory were discontinued at the end ot the 

 year 184-5, but tin; instruments were still allowed to remain, tliat 

 the portable instruments might be occasionally compared with them 

 during the Survey. 



