606 



Fourier's Theory of Heat. 



water on their surface, being capable of producing very sensible 

 differences. 



It is our ignorance of these latter circumstances which prevents 

 our being able to assign, precisely, the temperature of each planet. 

 All that we could do would be to determine, in a closely approxima- 

 ting manner, the degree of heat which the terrestrial globe would 

 acquire, if placed in their respective situations. But with regard to 

 the bodies situated at the extremities of the solar system, there is no 

 longer any uncertainty. The impression of the Sun's rays upon 

 these planets being extremely feeble at such a great distance, we 

 may be assured, that the temperature of their surface is but very 

 little above that of the planetary spaces, and consequently, that it is 

 subject to a degree of cold incompatible with the existence of life, 

 such as we see on the earth. This result is particularly evident 

 with regard to Uranus, which being 660 millions of leagues distant 

 from the sun, can derive no heat from its rays. 



These considerations suffice to shew how much Buffon deviated 

 from the truth in his conjectures upon the past, present, and future 

 state of the temperature of the planetary bodies. The errors into 

 which he fell, proceeded, 1st, from his being completely mistaken, 

 as to the rapidity of the total cooling of the heated masses. He was 

 led to suppose this rapidity to be incomparably greater than it really 

 is. Thus he allows only 4000 years for the earth to pass from the 

 temperature of boiling water to that which it now has ; whereas 4000 

 years would not be sufficient to reduce this temperature the tenth of 

 a degree. We may add, that he was not acquainted with that law of 

 cooling, by means of which a body, with a volume as large as that 

 of the planetary body, must necessarily be for a long time cooled at 

 its surface, while its interior is still in a state of ignition. 



2ndly. — From his assigning to the solar rays a power much too 

 limited. Thus whilst he supposes that our earth will become unin- 

 habitable, as soon as, by the evaporation of its internal heat, it shall 

 be reduced to that only which would accrue to it from the sun, it is 

 proved, on the contrary, that the heat which comes from this latter 

 source is now nearly all that influences our climates, and that it will 

 suffice to maintain them constantly the same, for an immense space 

 of time. In order to produce any sensible change in our climates, 



