950 
increase appears to be 1° Fahr., for 50 or 60 English 
feet of descent. Fourier first undertook to enquire, 
Whether, supposing the earth to have no primitive 
internal heat, the continual action of the sun might 
not produce the increasing temperature observed. 
But he found, on the contrary, that there is no such 
tendency, and that after a long time, the temperature 
at any moderate depth below the surface (but be- 
yond the yarying influence of the seasons) will be 
constant. Now, observations on the temperature of 
mines and Artesian springs extend to the depth of 
1700 English feet, and the influence of the seasons 
is usually quite extinct (in this latitude) at 50 or 60 
feet. The nearly uniform increase beyond is there- 
fore due to a source of heat within, or, what amounts 
to the same thing, to the relatively warmer state of 
the nucleus. The analysis of Fourier shows, that 
the variation of temperature in the successive strata 
of a sphere, cooling with excessive slowness, is very 
closely allied to the flux of heat which passes through 
it, and which is spent by radiation or otherwise at the 
Its insigni- surface. The result is exceedingly striking, and may 
ficant effect 
on climate. 
(677.) 
Depth of 
the fluid 
nucleus. 
(678.) 
Cause of 
secular 
change of 
climate. 
be considered as remarkably well established ; the 
flow of heat from the interior contributes to raise 
the temperature of the surface by only one seven- 
teenth of a degree of Fahrenheit ; or would melt an- 
nually a stratum of ice 7,th inch in thickness. This 
is all the refrigeration which the earth’s surface can 
ever suffer on this account, and in its present state 
of cooling, it would take millions of years even to 
reduce it by one half. So little ground is there for 
the belief of Buffon and his friends, who imagined 
that the destruction of animal and vegetable life must 
rapidly ensue by reason of the diminishing central 
heat. 
Again, the depth to which we must descend in 
order to reach a temperature sufficient for the supply 
of molten lava, is not excessive: for this depends 
entirely on the conductivity of the earth’s crust, 
which we know to be very small, A familiar in- 
stance occurs in streams of recent lava, where a crust 
soon forms, on which a man may walk safely, yet 
only be separated by a foot or less from the fiery 
liquid. Fourier calculated that the temperature of 
incandescence may prevail at a depth within the 
earth of only about 15 English miles, without affect- 
ing the superficial temperature by more than a small 
fraction of a degree. 
As it seems scarcely possible to ascribe some, at 
least, of the geological effects already mentioned to a 
cause of which the variations are imperceptible in 
such vast periods, other writers have suggested dif- 
ferent possible explanations, One which appears to 
have considerable probability, is that of Arago, that 
the heat emitted by the sun has a secular rate of 
change; in other words, that our sun is a variable star. 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. VI. 
Secondly, As to the effects of the sun’s heat on (679.) 
the earth. 
It has been already stated that Fourier E#ect of 
the sun’s 
showed that below a moderate depth, the heat of the heat on the 
earth would be uniform and invariable, so far as solar earth. 
radiation is concerned, At smaller depths, it will 
vary according to the season of the year, and these 
variations will at all depths be gone through in the 
same period with the variation at the surface, which, 
is of course annual, but the inflections follow a pe- 
culiar law in each latitude and climate, and even in 
one year compared with another. Within the depth 
to which the influence of the seasons extends, the 
amount of the range of temperature continually di- 
minishes, nearly in a geometrical progression, and at 
length it becomes insensible, At increasing depths, 
the periods of maxima and minima are continually 
retarded, so that at a certain depth, the earth is hot- 
test in winter and coldest in summer. <A smaller 
fluctuation of the same kind, and penetrating to a 
smaller depth, follows the diurnal range of tempe- 
rature. The manner in which these interesting phe- 
nomena are connected with the conducting power and 
specific heat of the earth’s crust, was clearly pointed 
out by Fourier, 
Before as well as after these theoretical investi- 
(680.) 
gations, observations on thermometers sunk to dif- Theory 
ferent depths in the ground had been made, and the compared 
results confirm these conclusions of theory in every sotey 
respect. Such observations were made by De Saus- 
sure, Leslie, Arago, Quetelet, and other experimen- 
ters, including the writer of these pages, who caused 
careful experiments to be made for five years, on 
three sets of thermometers, placed in different kinds 
of soil and rock, extending to a depth of above 25 
feet. By means of these, the conductivity of those 
various soils was determined with accuracy, and data 
afforded for subsequent enquiry. 
th expe- 
Amongst the most interesting enquiries connected (gg1.) 
with this subject, is the total quantity of heat re- Totalquan- 
ceived in a year from the sun by the absorptive MY, Hallmrsed 
action of the earth’s surface; but this does not ap- a. cael y 
pear as yet to have been successfully answered. from the 
Nevertheless, M. Pouillet, from merely experimen- 8%. 
tal data, considers that he has proved that the 
amount of solar radiation which reaches the earth 
would melt in a year an average thickness of 31 
métres of ice, all over its surface, provided that it 
were entirely absorbed. Poisson has obtained a re- 
sult four times less, and estimates the effect of the 
sun in raising the temperature of the climate of Paris 
at 24 centigrade degrees, a result incredibly small. 
Thirdly, On the temperature of space. Fourier 
was probably the first who introduced this idea into p 
(682.) 
he tem- 
science, The term perhaps is a doubtful one and perature of 
liable to misconception, but the object of the enquiry *P¢e— 
admits of being precisely stated. The researches of 
1 See Transactions of the Royal Society of Ldinburgh, vol. xyi., where a history is given of the observations previously 
made. 
