NATURAL PHILOSOPHY. 155 



therefore 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 lied on which they rest show the uninter- 

 rupted emission of heat from the interior toward the exterior of the 

 earth; ami the question is, Has tin- earth in 20 centuries actually lost 

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

 than 6,000,000 of meters only 15 centimeters ? 



In answering this question, three points enter into our calculation : 

 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 favor of some preconceived 

 opinion. 



Considering what is known about the expansion and contraction of 

 solids and liquids by heat and cold, we arrive at the conclusion that 

 for a diminution of 1 in temperature, the linear contraction of the 

 earth cannot well be less than yyyVoT P art 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. 



If we accept Laplace's result, that the length of a day has remained 

 constant during the last 2,500 years, and conclude that the earth's 

 radius has not diminished 1^ decimeter in consequence of cooling, 

 we are obliged to assume, according to the premises stated, that the 

 mean temperature of our planet cannot have decreased -4 g-g- in the 

 same period of time. The volume of the earth amounts to 2,b'50,000,- 

 000 of cubic miles. A loss of heat sufficient to cool this mass j|-j 

 would be equal to the heat given off when the temperature of 6,150,- 

 000 cubic miles of water decreases 1 ; hence the loss ibr one day 

 would be equal to 6'74 cubic miles of heat. 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 

 meters as the basis of his calculations, this celebrated mathematician 

 finds the heat which the globe loses by conduction through its crust 

 in the space of 100 years to be capable of melting a layer of ice three 

 meters in thickness and covering the whole surface of the globe ; this 

 corresponds in one day to 7 '7 cubic miles of heat, and in 2,500 years 

 to a decrease of 17 centimeters in the length of the radius. 



According to this, the cooling of the globe would be sufficiently 

 great to require attention when the earth's velocity of rotation is con- 

 sidered. At the same time it is clear that the method employed by 



