REPORT ON ELECTRICITY^ MAGNETISM, AND HEAT. 31 
regions. In the actual condition of the earth, acted on as it is 
by the sun, we have alternations in the influx and efflux of heat, 
but on the whole the same result as by the above theoretical 
view; an influx in the equator parts, an efflux at the poles, a 
circulation in the interior, and a diminution of the mean tem¬ 
perature in proceeding towards either pole* * * § . The formulae 
which represent the empirical laws of the dependence of the 
mean temperature on the latitude, have been stated by Professor 
Forbes in his Report on Meteorology (p. 215) ; namely, the old 
one of Mayer, which made the temperature nearly proportional 
to the square of the cosine of the latitude ; and other more exact 
rules since proposed by Brewster and others. I am not aware 
that any attempt has been made to bring these rules into ac¬ 
cordance or even comparison with Fourier’s theoretical for- 
mulie. Such an accordance wrnild be extremely interesting in 
its bearing on the theory. 
It is curious to obtain from the theory the actual amount of 
the solar heat annually poured upon the earth. This is statedf 
to be sufficient to melt a coat of ice 14 metres thick, encrusting 
the whole globe of the earth. 
( b .) The subject of Central Heat is of great interest to the geo¬ 
logist; and, as we have already seen, is illustrated by the 
theory. It appears, that if there be an increase of temperature 
in descending, such a fact can result from nothing but a central 
heat independent of existing influences. The discussion of the 
evidence of this fact must be left to the geological speculator; 
but we may here mention some of the results of theory which 
are fitted to make less formidable the idea of having avast abyss 
of incandescent matter within the comparatively thin crust of 
earth on which man and his w T orks are supported. It results 
from Fourier’s analysis!, that at 20,000 or 30,000 metres deep 
the earth may be actually incandescent, and yet that the effect 
of this fervid mass upon the temperature at the surface may be 
a scarcely perceptible fraction of a degree. The slowness with 
which any heating or cooling effect would take place through a 
solid crust is much greater than might be supposed. If the 
earth below 12 leagues’ depth were replaced by a globe of a tem¬ 
perature 500 times greater than that of boiling water, 200,000 
years would be required to increase the temperature of the sur¬ 
face by 1 degree§. A much smaller depth would make the ef- 
* Fourier’s formula is v = cos. xje y cos ° r dr, where cos. x is the sine an &y 
the cosine of the latitude, and the integral is taken from r = o to r = tt, (Fou¬ 
rier, Mem. Inst., tom. v. p. 173.) 
f Pouillet, tom. ii. p. 704. % Bullet, des Sci. 1820, p. 58. 
§ Mem. List. vii. p. 603. 
