24 THE POPULAR SCIENCE MONTHLY. 



instance it ought to produce a very observable effect upon the mo- 

 tions of the planet Mercury an effect not yet detected.* 



For this reason astronomers generally, while conceding that a por- 

 tion, and possibly a considerable fraction, of the solar heat may be ac- 

 counted for by this hypothesis, are disposed to look further for their 

 explanation of the principal revenue of solar energy. They find it in 

 the probable slow contraction of the sun's diameter, and the gradual 

 liquefaction and solidification of the gaseous mass. The same total 

 amount of heat is produced when a body moves against a resistance 

 which brings it to rest gradually, as if it had fallen through the same 

 distance freely and been suddenly stopped. If, then, the sun does con- 

 tract, heat is necessarily produced by the process ; and that in enor- 

 mous quantity, since the attracting force at the solar surface is more 

 than twenty-seven times as great as gravity at the surface of the earth, 

 and the contracting mass is so immense. 



In this process of contraction, each particle at the surface moves 

 inward by an amount equal to the whole diminution of the solar 

 radius, while a particle below the surface moves less, and under a 

 diminished gravitating force ; but every particle in the whole mass 

 of the sun, excepting only that at the exact center of the globe, con- 

 tributes something to the evolution of heat. To calculate the precise 

 amount of heat developed, it would be necessary to know the law of 

 increase of the sun's density from the surface to the center ; but 

 Helmholtz, who first suggested the hypothesis, in 1853, has shown 

 that, under the most unfavorable suppositions, a contraction of about 

 250 feet a year in the sun's diameter a mile in twenty-one years 

 would account for its whole annual heat-emission. This contraction is 

 so slow that it would be quite imperceptible to observation. It would 

 require 9,500 years to reduce the diameter a single second of arc (since 

 1" equals 450 miles at the sun's distance), and nothing less would be 

 certainly detectable. 



Of course, if the contraction is more rapid than this, the mean 

 temperature of the sun must be actually rising notwithstanding the 

 amount of heat it is losing. Observation alone can determine whether 

 this is so or not. 



If the sun were wholly gaseous, we could assert positively that it 

 must be growing hotter ; for it is a most curious, and at first sight 

 paradoxical, fact, first pointed out by Lane in 1870, that the tempera- 

 ture of a gaseous body continually rises as it contracts from loss of 

 heat. By losing heat it contracts, but the heat generated by the con- 

 traction is more than sufficient to keep the temperature from falling. 



* Leverrier considered that he had detected in the motions of Mercury an irregularity 

 of the kind indicated, but much smaller. It was such as, according to his calculations, 

 would be accounted for by the action of one or several planets, whose aggregate mass 

 should be much less than that of the earth. It was on this basis that he founded his 

 strong belief in the existence of the intra-mercurial planet, Vulcan. 



