66 PROFESSOR W. THOMSON ON THE 



cannot do, that its influence is insensible. It is in fact not only proved to exis 

 as a cause of solar heat, but it is the only one of all conceivable causes which we 

 know to exist from independent evidence. 



To test the possibility of this being the principal or the sole cause of the phe- 

 nomenon, let us estimate at what rate meteoric matter would have to fall on the 

 . v un, to generate as much heaths 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 Earth's 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 „„„, ., , 



•06 x ( ^ 41 ouo ) = 2 78l 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 



83-4 x (^.°00.00OX ' = 386,900 ft. 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 shewn dynamically (the 

 velocity of rotation of the Suns 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 infi- 

 nite 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 



12 x 28 x 32-2 x 441,000 nnn ., , , .. . . , 



,,J ^ro — = 390 mL les per second ; and its mechanical 



value per pound of meteoric matter is 



28 x 441,000 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 

 000060 pounds per second (or about a pound every five hours.) At this rate 

 the surface would be covered to a depth of thirty feet in a 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 Sun.f A greater rate of deposit than this could not be required, 

 if the hypothesis of no resistance, except in the locality of resistance with lumi- 

 nous reaction, were true ; but a less rate would suffice if, as is probable enough, 



* Memoire sur la Chaleur Solaire, &c, Paris 1838 ; See Comptes Rendus, July 1838 ; or 

 Pouillet, Traite de Physique, vol. ii. 



"j" This is rather more than double the estimate Mr Waterston has given. The velocity of 

 impact which he has taken is 545 miles per second, in the calculation of which, unless I am mistaken, 

 there must be some error. 



