Dr. J. R. Mayer on Celestial Dynamics. 423 



is as little able to stop the cooling-tendency of the earth as the 

 moderate warmth of the air can prevent the cooling of a red-hot 

 ball suspended in a room. 



Many phenomena, for instance the melting of the glaciers 

 near the bed on which they rest, show the uninterrupted emis- 

 sion of heat from the interior towards the exterior of the earth ; 

 and the question is, Has the earth in 25 centuries actually lost 

 no more heat than that which is requisite to shorten a radius of 

 more than 6 millions of metres only 15 centimetres ? 



In answering this question, three points enter into our cal- 

 culation : — (1) the absolute amount of heat lost by the earth in 

 a certain time, say one day ; (2) the earth's capacity for heat ; 

 and (3) the coefficient of expansion of the mass of the earth. 



As none of these quantities can be determined by direct 

 measurements, we are obliged to content ourselves with probable 

 estimates ; these estimates will carry the more weight the less 

 they are formed in favour of some preconceived opinion. 



Considering what is known about the expansion and contrac- 

 tion of solids and liquids by heat and cold, we arrive at the con- 

 clusion that for a diminution of 1° in temperature, the linear 

 contraction of the earth cannot well be less than 100 ^th part, a 

 number which we all the more readily adopt because it has been 

 used by Laplace, Arago, and others. 



If we compare the capacity for heat of all solid and liquid 

 bodies which have been examined, we find that, both as regards 

 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*. 



* The capacity for heat, as well as the coefficient of expansion of matter, 

 as a rule, increases at higher temperatures. As, however, these two quan- 

 tities act in opposite ways in our calculations, we may be allowed to 

 dispense with the influence which the high temperature of the interior of 

 the earth must exercise on these numbers. Even if, in consequence of 

 the high temperature of the interior, the earth's mass could have a ca- 

 pacity two or three times as great as that which it has from 0° to 100°, it 

 is to be considered, on the other hand, that the coefficient of expansion, 

 1^7555, only holds good for solids, and is even small for them, whilst in 

 the case of liquids we have to assume a much greater coefficient : for 

 mercury between 0° and 100°, it is about six times as great. Especially 

 great is the contraction and expansion of bodies when they change their 

 state of aggregation ; and this should be taken into account when con- 

 sidering the forma'uon of the earth's crust. 



