608 



Fourier's Theory of Heat. 



must have been much more rapid ; but from the most remote his- 

 torical times, all these great phenomena relative to the earth have 

 assumed a character of stability which is very remarkable. It is 

 rigorously demonstrated that, from the time of the Greek school of 

 Alexandria to the present day, the temperature of the terrestrial sur- 

 face has not diminished, in consequence of the declining of its inter- 

 nal heat, the 300dth part of a degree of heat of the terrestrial globe. 



From these different reflections, we conclude that, after having 

 diminished for an immense length of time, the influence of the in- 

 ternal heat of the globe, however intense it may be, produces only 

 an imperceptible effect on the surface ; and that this effect, feeble as 

 it is, will not, however, be totally destroyed till after an unlimited 

 time, for strictly speaking, it will become more and more feeble, un- 

 til the internal heat be wholly dissipated. 



Although the effect of the interior heat may not be any longer 

 sensible at the surface of the earth, the total quantity dissipated in 

 a given time, as a year or a century, can be measured; and M. 

 Fourier, who has ascertained it, has shewn that it was once more 

 considerable. That which traverses a square metre in superficies, 

 and is in the course of a century dispersed into the planetary spaces, 

 could melt a column of ice which should have for its base this square 

 metre, and a height of about three metres. 



The same geometrician has ascertained the quantity of heat, the 

 oscillations of which determine the alternation of the seasons, for 

 every point of the globe. This quantity, supposing the terrestrial 

 envelope to be made of forged iron, would be, for every square 

 metre of superficies, equivalent to that which would melt a cylindri- 

 cal column, having for its base this square metre, and three metres 

 of height : that is to say, that the quantity of heat which every year 

 produces the alternation of the seasons, would be on this supposi- 

 tion, perceptibly equal to that which the terrestrial globe loses in a 

 century, in consequence of the evaporation of its internal heat : but 

 the envelope of the terrestrial globe being formed of substances, 

 which conduct the heat in a much less degree than forged iron 

 would do, the annual loss is really less considerable. 



It is of great importance to observe, that the mean temperature 

 may experience, from accidental causes, variations incomparably 



