1816.} during the Year 1815. 25 
not remain at the centre, but must diffuse itself equally through 
the whole globe; so that in time the surface of the earth would 
become as hot as the centre. To this Mr. Playfair answered, that 
the alleged equalization would undoubtedly take place, provided 
heat did not make its escape from the surface of the earth; but if 
the heat (as was probable) escaped from the surface of the earth as 
fast as it was conveyed to it from the centre, then no accumulation 
or increase of temperature would take place. The principal object 
of the paper in question, is to show that no such escape of heat 
takes place from the surface of the earth; that nature has guarded 
against it effectually by the constitution of the atmosphere; that 
heat, in consequence, is constantly accumulating on this globe of 
ours ; and that the time will come, when the whole surface of the 
earth shall have acquired the same temperature. According to this 
notion the polar regions are becoming annually warmer, and will in 
time equal the heat of the torrid zone. 
But I conceive that our ingenious author has committed an over- 
sight, by greatly underrating the effect of the radiation of heat. 
Wheever will peruse with attention Dr. Wells’s admirable Essay on 
Dew, will be satisfied that this radiation, even in cold weather, is 
very considerable. On clear nights he often -found the grass 13° or 
14° colder than the atmosphere. There is no reason for supposing 
that heat radiated in this manner is intercepted by the atmosphere. 
As we have no evidence that the polar regions are warmer at present 
than they were some thousand years ago, I do not see why we 
should believe that any such amelioration has.taken place. Indeed 
the evidence is rather on the contrary side ; for North Greenland, 
which formerly was accessible and even colonized by the Danes, 
has been for ages blocked up with ice, so that we are ignorant 
whether the wretched inhabitants still struggle with their situation, 
or have perished from want of food and by the inclemency of the 
climate. Mr. Scoresby has ascertained that the mean temperature 
of latitude 78°, on the coast of Spitzbergen, is only 18°, instead 
of 34°, as calculated by Mr. Kirwan; and he thinks it demon- 
strable, that at the pole the mean temperature will not exceed 7° 
or 8°. It is much more probable that the heat thrown from the 
surface of the earth by radiation is nearly equivalent to what is 
thrown into it by the solar rays, and that the mean temperature of 
the globe remains nearly stationary. We have other proofs against 
the existence of a central fire, which appear quite conclusive. The 
mean temperature of mines is always found to be the mean tempe~ 
rature of the country in which the mine in situated. No increase 
of temperature has ever been observed to take place as the mine 
increases in depth, yet this ought undoubtedly to happen on the 
supposition of the existence of a central fire, even according to the 
reasoning of Mr, Playfair himself. 
Il, SIMPLE SUPPORTERS, 
The simple supporters of combustion, at present known, amount 
a 3 
