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 
Sun, to generate as much heats is emitted. According to PourLier’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 
06 (aaa 2 
441,000 = 2781 thermal units per second 
per square foot of his surface. 
The mechanical value of this (obtained by multiplying it by JouLz’s equivalent, 
1390) is 
ad a ell ir ft. Ibs: 
441,000 386,900 ft. Ibs 
Now if, as Mr WaTERSTON supposes, a meteor either strikes the Sun, or enters an 
atmosphere where the luminous and thermal excitation takes place, wthout having 
previously experienced any sensible resistance, it may be shewn 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 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 
ine x 28 x 32°2 x 441,000 
5280 
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 
Warerston’s form of the Gravitation Theory, to strike the Sun per square foot is 
0:000060 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.+ 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, 
2 
ad — 
= 390 miles per second; and its mechanical 
* Mémoire sur la Chaleur Solaire, &c., Paris 1838; See Comptes Rendus, July 1838; or 
Povitier, Traité de Physique, vol. ii. 
+ This is rather more than double the estimate Mr Warsrston 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. 
a es 
