410 J. R. Mayer on Celestial Dynamics. 
volume and weight, the capacity of water is the greatest. Even 
the gases come under this rule; hydrogen, however, forms an 
exception, it having the greatest capacity for heat of all bodies 
when compared with an equal weight of water. In order not 
to take the capacity for heat of the mass of the earth too small, 
we shall consider it to be equal to that of its volume of water, 
which, when calculated for equal weights, amounts to 0°184.' 
If we accept Laplace's result, that the length of a day has re- 
mained constant during the last 2500 years, and conclude that 
the earth’s radius has not diminished 14 decimetre in conse- 
quence of cooling, we are obliged to assume, according to the 
remises stated, that the mean temperature of our planet cannot 
ave decreased ;1,° in the same period of time. 
The volume of the earth amounts to 2650 millions of cubic 
miles. A loss of heat sufficient to cool this mass z},° woul 
be equal to the heat given off when the temperature of 6,150,000 
eubic miles of water decreases 1°; hence the loss for one day 
- Fourier has investigated the loss of heat sustained by the 
earth. Taking the observation that the temperature of the earth 
increases at the rate of 1° for every 30 metres as the basis of his 
calculations, this celebreted mathematician finds the heat which 
responds in one day to 7°7 cubic miles of heat, and in 2500 
years to a decrease of 17 centimetres in the length of the radius. 
According to this, the cooling of the globe would be suffi- 
which is annually lost by the earth; for simple conduction 
through terra firma is not the only way by which heat escapes 
m our globe. 7 
_ In the first place, we may make mention of the aqueous de- 
posits of our atmosphere, which, as far as they penetrate our 
1 The capacity for heat, as well as the coéfficient of expansion of matter, a8 ® 
Tule, increases at higher temperatures. As, however, these two uantities act in 
opposite ways in our calculations, we may be allowed to dispense with the influence 
which the high teuiperature of the interior of the earth must pong t 
from 0° to 100°, it is to be considered, on the other hand, that the coéfficient of ex 
f 2 ansi Mea © fs - soli } hem, whilst 
100°, it is about six times as great. Especially great is the contin 
d expansion of bodi ' change their state of aggregation; and 
¢ taken into account when considering the formation of the earth's crust. 
tween 0° 
