Mechanical Energies of the Solar System. 413 



as is emitted. According to Pouillet^s data*, '06 of a thermal 

 unit Centigrade is the amount of heat incident per second on a 

 square foot directly exposed to solar radiation at the eartVs 

 distance from the sun_, which being 95,000,000 miles, and the 

 sun^s radius being 441,000 miles, we infer that the rate of 

 emission of heat from the sun is 



^^ /95,000,000\2 ^^^^ XT, 1 -^ 

 •06 X ( 441 000 ) =^^^1 thermal units per 



second per square foot of his surface. 



The mechanical value of this (obtained by multiplying it by 

 Joule^s equivalent, 1390) is 



83-4x(?^^00^y=386,900ft.lbs. 

 V 441,000 / ' 



Now if, as Mr. Waterston supposes, a meteor either strikes the 



sun, or enters an atmosphere where the luminous and thermal 



excitation takes place, without having previously experienced any 



sensible resistance, it may be shown dynamically (the velocity of 



rotation of the sun^s surface, which at his equator is only a mile 



and a quarter per second, being neglected) that the least relative 



velocity which it can have is the velocity it would acquire by , 



solar gravitation in falling from an infinite distance, which is 



equal to the velocity it would acquire by the action of a constant 



force equal to its weight at the sun^s surface, operating through a 



space equal to his radius. The force of gravity at the sun^s surface 



being about 28 times that at the earth's surface, this velocity is 



2 X 28 X 32-2 X 441,000 ^^_ ., , . 

 ^-^qt: ' = 390 miles per second; and 



its mechanical value per pound of meteoric matter is 



28 X 44] ,000 X 5280 = 65,000,000,000 ft. lbs. 

 Hence the quantity of meteoric matter that would be required, 

 according to Mr. Waterston's form of the gravitation theory, 

 to strike the sun per square foot is 0*000060 pound per second 

 (or about a pound every five hours). At this rate the surface 

 would be covered to a depth of 30 feet in the year, if the 

 density of the deposit is the same as that of water, which is a 

 little less than the mean density of the sunf. A greater rate 

 of deposit than this could not be required, if the hypothesis of 

 no resistance, except in the locality of resistance with luminous 

 reaction, were true ; but a less rate would suffice if, as is pro- 

 bable enough, the meteors in remote space had velocities relative 



* Memoire sur la Chaleur Solaire, &c., Paris 1838. See Comptes Ren- 

 dus, July 1838; or Pouillet, Traits de Physique, vol. ii. 



t This is rather more than double the estimate Mr. Waterston has given. 

 The velocity of impact which he has taken is 645 miles per second, in the 

 calculation of which, unless I am mistaken, there must be some error. 



^/ 



