Tlie Maintenance of Energy. 13 



explain facts. While the geologist can apparently furnish 

 satisfactory reasons within the limits of his science, his 

 conclusions clash with the requirements of physical 

 astronomy, and vice versct. 



The nebula theory of Kant and Laplace is, that all the 

 materials out of which the bodies of our solar system were 

 formed were in the beginning of things resolved in their 

 original elements, and filled all the space of the universe in 

 which these bodies now move. This nebulous mass was in an 

 intensely heated gaseous condition. It condensed towards 

 the centre, and in doincj so threw off successive lings which 

 on further disruption and condensation assumed the form 

 of planets. The sun himself is now supposed b}^ most 

 astromers to be almost altogether gaseous to the centre 

 (Young.)* 



One of the most interesting questions of cosmical physics 

 is how the sun has been radiating heat into space for 

 millions of years without any apparent diminution of the 

 supply. How is this continued outpouring of heat 

 maintained ? If we calculate at what rate the temperature 

 of the sun would be annuall}^ lowered by the radiation from 

 its surface, we find it to be 2|° F. per annum, supposing its 

 specific heat to be that of water, and from 5° to 10° F. per 

 annum, if we suppose it to be the same as most of the 

 substances which compose our globe (Young). It would 

 entirely cool oft* in a few thousands of years after its 

 formation if it had no other source of heat than that shown 

 by its temperature. It has been attempted to explain the 

 source of its maintenance of heat in two ways. The first is 

 known as the meteoric theory and it is — that the heat of the 

 sun is kept up by the impact of meteors upon his surface. It 

 is considered that though the sun may at some past time 

 have received a large supply of heat in this way, the 

 quantity of meteoric matter now falling is totally inadequate 

 to keep up the supply. The second is the contraction theory 

 which appears to be accepted by most astronomers. As the 

 sun's globe cools off" it must contract, and the heat generated 

 by this contraction Avill suftice to make up the entire loss. 

 Knowing the amount of energy which the sun annually 

 radiates in the form of heat, it has been calculated from the 

 mechanical equivalent of the heat thus radiated, by what 

 amount it must contract to make it up. With the present 



* The Sun. International Scientific Series. 



