30 
FIFTH REPORT— 1835. 
and relations in question, and the possibility of tracing them 
mathematically, they would have brought a far wider circle of 
intellect to bear upon the inquiry; and thus would have tended 
much to the diffusion of sound knowledge, and, not improbably, 
to the promotion of further discovery. Even now, any mathe¬ 
matician who would present the subject to students in such a 
generally accessible form, would probably find himself rewarded 
both by the simplicity and elegance of the propositions to which 
he would be led, and by the utility which his labours would be 
found to possess. 
4. The Application of the theory of heat to questions which 
occur as important subjects of speculation in natural philoso¬ 
phy, is the last division of our task. We may notice as be¬ 
longing to this division three investigations : The effect of the 
Solar Heat upon the Earth, and the laws of its Distribution ; the 
effect of the Primitive Central Heat, and the evidence of its ex¬ 
istence ; the effect of the Proper Heat of the Planetary Spaces, 
and the evidence that such heat exists. 
(a.) With regard to the first point, I have already stated the 
conclusions respecting the propagation of alternations of heat 
and cold in a solid; and I have now only to add, that several 
observers have ascertained by experiment that the propagation 
of the solar heat into the interior of the earth does follow the 
rules which theory points out; namely, that the range of oscil¬ 
lation of temperature becomes narrower and narrower as we 
recede from the surface; that the annual oscillations are sen¬ 
sible to a much greater depth than the-diurnal; and that the 
maximum heat and maximum cold occur later below than at 
the surface, and later and later the deeper we descend. These 
rules, in the whole or in part, were traced by Ott at Zurich in 
1J62, by Saussure in 1785, by Hemmschneider at Strasburg 
in 1821 and the two following years, but most completely by 
Leslie in 1816 and 1817*- These observers and others have 
found that at a certain depth, as 40, 50, or 60 feet, the tempe¬ 
rature ceases to show the effect of the changes of solar influence ; 
we have an invariable stratum : at smaller depths the rules 
are such as have been stated. It is, moreover, to be observed, 
that the temperature of the invariable stratum is different in 
different places; and if we assume, as an approximation to the 
actual condition of the earth, that the equator is kept at a con¬ 
stant high temperature, we shall have a constant flow of heat 
in the interior of the sphere from the equatorial to the polar 
* Pouillet, Elemens de Meteorologie, tom. ii. p. 643 ; where Leslie’s experi¬ 
ments, made in the grounds of Mr. Ferguson of Raith, are erroneously attri¬ 
buted to that gentleman. 
