152 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



increase appears to be lFahr., 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 varying 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 k e 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 3*5 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. 



(677.) Again, the depth to which we must descend in 

 thTfluid or der to reach a temperature sufficient for the supply 

 nucleus, 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. 



(678.) As it seems scarcely possible to ascribe some, at 

 Cause of j eas ^ o f the geological effects already mentioned to a 

 change of cause of which the variations are imperceptible in 

 climate. 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. 



Secondly, As to the effects of the sun's heat on (679.) 

 the earth. It has been already stated that Fourier *J ffect ^ 

 showed that below a moderate depth, the heat of the nea t on ' 

 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- ,g 80 , 

 gations, observations on thermometers sunk to dif- Theory 

 ferent depths in the ground had been made, and the compare 

 results confirm these conclusions of theory in every ^ ltb ex l 

 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. 1 



Amongst the most interesting enquiries connected (681.) 

 with this subject, is the total quantity of heat re- Total qui 

 ceived in a year from the sun by the absorptive ili J . of ^ 



. r6C61V6Cl 



action of the earth's surface ; but this does not ap- the eart ] 

 pear as yet to have been successfully answered, from th 

 Nevertheless, M. Pouillet, from merely experimen- sun - 

 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 

 metres 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 / 6 p 2 \ 

 was probably the first who introduced this idea into The ten* 

 science. The term perhaps is a doubtful one andperature 

 liable to misconception, but the object of the enquiry s P ace ~ 

 admits of being precisely stated. The researches of 



1 See Transactions of the Royal Society of Edinburgh, vol. xvi., where a history is given of the observations previously 

 made. 



