A. Vaughan — Problem of a Cooling Earth. 509 



rock would solidify at a very mucli higher temperature than at the 

 surface., The average value assumed must represent the temperature 

 at a great depth ; in fact, the constant multiplier of the integral, 

 which occurs in the equation connecting temperature, depth, and time, 

 is found by assuming that the temperature of solidification is the 

 average value which it has at a very great depth. It is, then, quite 

 beside the question to employ the temperature of solidification of 

 slag or other molten rock at the surface, for we really require to 

 know at what temperature the rock would solidify under a pressure 

 of, say, a hundred miles of rock. At the surface molten slag 

 solidifies at about 3000° F. ; it seems then, that the assumption of 

 7000° F. for use in the problem under consideration, leans very 

 much on the side of under-estimation. 



Again, the conductivity or, in other words, the number which 

 represents the amount of heat which escapes through a given thick- 

 ness of rock in a given time is assumed as 400°, a number obtained 

 by experiments on rocks at the surface. But the greater the 

 temperature, the greater must be the instability of the mean centre 

 of oscillation for each molecule, and this instability must increase 

 until the fusion point is reached, when the molecules have free 

 motion over each other. Hence, the hotter the rock, the more heat 

 will be absorbed in molecular action, and, consequently, the less 

 will be the heat conducted through in any given time ; or, in other 

 words, it seems probable that the conductivity must diminish as we 

 reach a greater depth beneath the surface. It follows, that the average 

 value of the conductivity must be much less than its surface value. 

 But any decrease in this value increases the time in the same ratio. 

 A further cause acting in the direction of increasing the time has 

 already been pointed out, though probably of no great weight and 

 not worthy of consideration, beside the corrections suggested above. 

 This consists in the conduction being diminished when heat has to 

 traverse rocks stratified or laminated transversely to the flow of 

 heat. Since the rocks near the surface have their separation planes, 

 as a general rule, inclined at a low angle, the stratified rocks of the 

 crust must conserve the heat passing through by conduction ; though 

 this will be, to a small extent, counteracted by the existence of 

 vertical joints which allow a more rapid loss by a kind of surface 

 creep. 



In fine, the geologist and biologist have a right to demand of 

 the physicist first, that all losses of heat, other than those considered 

 in his calculations, should be shown to have a negligible effect on 

 the result ; and, secondly, that the numerical data he assumes 

 should be at least greater than the probability, not less. Results 

 obtained by the substitution of values, formed on considerations of 

 phenomena taking place under atmospheric pressure, and, as in the 

 case of the determination of conductivity, at low temperatures, 

 can lead to nothing but misconception. There seems no reason 

 why the estimate of 100 million years should not be increased to 

 1000 million, which would almost allow as much time as the 

 evolutionist desires. 



