FREE AND PERFECTLY ELASTIC MOLECULES IN A STATE OF MOTION. 55 
hammered, or compressed, or rubbed. Friction and every other expenditure of 
mechanical force gives birth to heat or molecular vis viva, which is dissipated by 
radiation and conduction. 
The observations of Sir John Herschel and Professor Forbes with the actinometer 
(‘ Phil. Trans.,’ 1842) have recently supplied a knowledge of the absolute value of the 
solar radiation before it passes through the atmosphere. They have found it to 
amount to 38S'4 actines, each actine being one-millionth of a metre in thickness of ice 
melted per minute. This is equal to ] '835 ft. of ice melted in a mean solar day. 
Now, since it is known that ice requires 140° of heat to melt it, and the mechanical 
value of 1° in water is equal to the weight of the water raised through 673 feet 
against the force of gravity on the Earth’s surface (§ 25), we have the means of 
computing exactly the absolute mechanical power of the solar radiation—the absolute 
force thrown out by the Sun in a given time. 
There are various ways of reckoning this and obtaining a clear conception of it 
with reference to different standards. The fundamental principles are contained in 
Section 3, and an example of their application is given in detail in § 25. The results 
of four computations are as follows :— 
1. During one year the solar force upon a square foot at the Earth’s mean distance 
from the Sun is equal to 20 tons raised 20 miles, or to about one ton raised one mile 
per day, which is equivalent to ^th of a horse-power, according to engineers’ mode of 
reckoning;. 
2 . At the Sun its amount in one year is equivalent to the descent of a stratum of 
the Sun’s surface (and of its mean density) 3^ miles thick through its own breadth. 
3. If the Sun is supposed to contract uniformly throughout its mass so that its 
radius becomes 3^- miles less in consequence of the general increase of density, the 
force generated is sufficient to supply the solar radiation for about 9000 years. 
4. If a mass equal to the Earth descended to the Sun’s surface from its mean 
distance, it would acquire a velocity of 390 miles per second, and the vis viva 
generated when it strikes the Sun would amount to the force thrown out by the Sun 
in 45 years. 
The density of the Sun being little more than that of water, it is possible that the 
mere gradual contraction of its bulk, or natural subsidence of the mass, may generate 
sufficient force to supply the amount of radiation without any diminution of tem¬ 
perature, and it would appear from the third computation that the decrement of the 
apparent diameter of the Sun owing to such condensation may not amount to more 
than - 5 - 0 -th of a second in 9000 years. 
Note M.—Barometric Formula. 
This may include the effect of aqueous vapour by the formula in Note B. Let 
r = mean dew point, t — mean temperature of the air at the two stations. Find 
