460 ON THE SOURCES OF HEAT WHICH INFLUENCE CLIMATE. 
mean temperature of the whole earth’s surface. These formulae determine with 
moderate exactness the temperature of places under the temperate zone, but 
Mayer’s begins entirely to fail for the higher latitudes, while ail of them are 
uncertain in their application to places within the tropics, and to the whole 
southern hemisphere; also since the computations of Fourier give to high 
latitudes a degree of cold far greater than these empirical formulae, I despaired of 
being able to assign a tolerably true mean temperature to the globe’s surface ; but 
this mean temperature will be higher than at first sight might appear, from the 
circumstance of a degree of latitude and longitude within the tropics enclosing an 
area so much larger than the same degrees when nearer the Pole. The value of 
a degree of latitude is invariably sixty geographical miles; but those of longitude 
regularly diminish from sixty miles on the equator, to fifty-two miles at latitude 
SO, to thirty miles at latitude 60, or half what they were on the equator, and one 
mile at 80 latitude. Hence it follows that a large proportion of the superficies of 
the globe is exposed to a tropical climate; one half of the earth’s surface is con¬ 
tained within 30 degrees of north and south latitude; therefore its mean tem¬ 
perature at the level of the sea greatly exceeds that of our climate. Meyer’s 
formulae would give it at 65, Farhenheit, but this is certainly too high. As 
already stated, it is in vain in the present extent of our meteorological know¬ 
ledge to endeavour to arrive at it correctly; but assuming 60, Fahrenheit, as 
the mean—which is as high as our data will warrant-—then let us also suppose 
the earth free from all central heat, and that it had existed a sufficient time for 
its mass to attain the mean temperature of its surface; then in penetrating into 
the earth, under any latitude, and being careful to prevent the heat evolved by 
the compulsion of the air from affecting the experiments, we should find the 
temperature slowly to approximate to the mean of the central mass, which in 
such a case would be the mean temperature of the whole superficies ; thus in high 
latitudes the heat would increase as we descend, while under the equator a 
diminution of heat would be expected. 
The latent caloric in the air, being reduced by its increased density, tends also 
greatly to elevate the temperature as we descend under every parallel, particularly 
where currents of air are circulating; but the increment of temperature, as we 
deepen our mines, is so great that it exhausts both of these recources, and, being 
further confirmed by experiments conducted with the greatest care, where no 
currents could vitiate the result, the elevation of temperature must be admitted 
as altogether independent of solar rays; and we have only left, to account for it, 
the existence of central heat coeval with the earth’s creation. 
The profound computations of M. Fourier led to this result; the experimental 
researches of M. Cordier and M. De La Reene tend towards similar conclusions ; 
