144 LECTURES 



100° additional of surface temperature to raise this to tropical heat. 

 This would necessitate a temperature of 2,000° at the depth of sixty- 

 feet, a condition of things, it would seem, utterly incompatible with 

 the existence of luxuriant vegetation on the surface. 



The second mode of accounting for it is by means of distribution of 

 land and water uj3on the earth's surface. Land, as compared with 

 water, is both a better absorber and better radiator of heat, i. e., will 

 both heat faster under the influence of a source of heat, as the sun, 

 and cool faster when that source is withdrawn. This is familiarly 

 illustrated by land and sea breezes. Again : the earth at the equator 

 receives more heat from the sun than it radiates, while at the poles, 

 on the contrary, it radiates more than it receives from the sun, the 

 overplus in both cases being balanced by the currents of ocean and 

 atmosphere. If these currents could be prevented, the equator, for a 

 time, would get progressively warmer, and the jooles progressively 

 colder. We may evidently, then, look upon the earth as a body heat- 

 ing at the equator and cooling at the poles. Now, when we recollect 

 the great absorbing and radiating power of land, as compared with 

 water, it is easy to see that the mean temperature of the earth's surface 

 may be materially affected by the distribution of these elements with 

 reference to the two points in question. For instance, if the water be 

 all collected in a belt about the equator, and the poles be occupied 

 entirely by land, we would have the conditions most unfavorable for 

 heating at the equator and most favorable for cooling at the poles. 

 The result would be a considerable lowering of the mean temperature. 

 If, on the contrary _, the waters be gathered into polar oceans, leaving 

 an equatorial belt of land, the conditions would be most favorable for 

 heating at the equator and most unfavorable for cooling at the poles, 

 and the mean temperature would consequently rise. It is estimated 

 that these two extreme conditions would bring down the mean tem- 

 perature to 32°, or raise it to tropical heat. It is not to be supposed 

 that such extreme conditions ever existed ; but any approximation to 

 such conditions — for instance, a decided predominance of land towards 

 the equator or poles — would produce the same effects to a corresponding 

 degree. Now, it is possible that the greater heat of the coal period 

 may be due to some such distribution of land and water. 



The fatal objection to this explanation is that we find no coal in 

 tropical regions. As every coal field presupposes a large river, and 

 therefore a considerable extent of land, the distribution of coal may 

 be looked upon as in a general way indicative of the distribution of 

 land during the period. It would seem from this that the larger 

 bodies of land existed in temperate and arctic rather than in tropical 

 regions. 



But if it is impossible by distribution of land and water to account 

 for the greater mean temperature, it is at least easy in this way to 

 account for the greater liumidity and uniformity of climate which we 

 have found equally to characterize this period. I have already alluded 

 to the fact that the pala?ozoic seas were probably very wide and the 

 land correspondingly small in extent and low, and that such a condi- 

 tion of things, on account of the very limited condensation and pre- 

 cipitation of vapor^ would produce a very humid climate. Now, water 



