LABOURS RELATIVE TO THE SOLAR SYSTEM. 289 
LABOURS RELATIVE TO THE SOLAR SYSTEM. 
Herschel occupied himself very much with the sun, 
but only relative to its physical constitution. The ob- 
servations that the illustrious astronomer made on this 
subject, the consequences that he deduced from them, 
equal the most ingenious discoveries for which the scien- 
ces are indebted to him. 
In his important memoir in 1795, the great astronomer 
declares himself convinced that the substance by the in- 
termediation of which the sun shines, cannot be either a 
liquid, or an elastic fluid. It must be analogous to our 
clouds, and float in the transparent atmosphere of that 
body. The sun has, according to him, two atmospheres, 
endowed with motions quite independent of each other. 
An elastic fluid of an unknown nature is being con- 
stantly formed on the dark surface of the sun, and rising 
up on account of its specific lightness, it forms the pores 
in the stratum of reflecting clouds ; then, combining with 
other gases, it produces the wrinkles in the region of 
Juminous clouds. When the ascending currents are 
powerful, they give rise to the nuclei, to the penumbre, 
to the facule. If this explanation of the formation of 
solar spots is well founded, we must expect to find that 
the sun does not constantly emit similar quantities of 
light and heat. Recent observations have verified this 
conclusion. But large nuclei, large penumbre, wrinkles, 
facule, do they indicate an abundant luminous and calo- 
rifie emission, as Herschel thought; that would be the 
result of his hypothesis on the existence of very active 
ascending currents, but direct experience seems to con- 
tradict it. 
The following is the way in which a learned man, 
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