Fourier s Theory of Heat. 



607 



whilst the surface of the earth remains the same, it would in reality- 

 be necessary, either that our sun should diminish in heat, or that 

 our entire solar system should be transported into a region of the 

 universe, in which the temperature of the planetary spaces would be 

 sensibly different from that in which we are immerged. 



BufFon attempted to indicate, in a precise manner, the time 

 which would necessarily be requisite for each planetary body to pass 

 from a state of fusion produced by heat, to a degree of cold incom- 

 patible with life. At the present day, in consequence of the theory 

 of heat, nothing would be so easy as to resolve this question in the 

 most precise manner ; and thus to determine the age of the planets, 

 if we had any mens of learning what was their initial temperature. 

 But not knowing this, we can determine nothing, and must content 

 ourselves with pointing out some results proper to give an idea of 

 the immense time which must have elapsed since the origin of our 

 planetary system. 



M. Fourier, in endeavouring to ascertain the periods of time 

 which similar solid bodies, similarly heated, would require to reduce 

 them to the same state when, after having been elevated to an equal 

 temperature, they should be immerged in the same medium, arrived 

 at this remarkable result : — that the earth once heated to any tem- 

 perature whatever, and plunged into a colder medium than itself, 

 would cool no more in 1,280,000 years, than a globe of a foot in 

 diameter formed of the like substances, and placed in the same cir- 

 cumstances, would in a second; that is to say, that in this really 

 immense time no appreciable variation would take place in its tem- 

 perature. We may see, by this result, with what slowness the ge- 

 neral changes take place in the interior of the planets. " The dura- 

 tion of these grand phenomena," says M. Fourier, " corresponds with 

 the dimensions of the universe ; it is measured by numbers of the 

 same order as those which express the distances of the fixed stars." 



Once familiarized with the ideas of these prodigious numbers, we 

 shall not be farther astonished to learn that whatever may be the in- 

 fluence exercised upon the surface of the ground by the internal heat, 

 this influence will last for an unlimited time ; and that more than 

 30,000 years will pass before it is reduced to the half of that which 

 it now is. In truth, at the commencement of things, the variations 



