HERSCHEL. 215 



motions quite independent of eacli other. An elastic fluid of an unknown 

 nature is being" constantly formed on the dark surlace 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 pro- 

 duces the striiB in the region of luminous clouds. When the ascend- 

 ing curi^ents are powerful, they give rise to the 7iuclei, to pemimbrm, and 

 thQ facnlcc. If this explanation of the formation of solar spots is well 

 founded, we must expect to find that the sun does not constantly emit 

 equal quantities of light and heat, llecent observations have verified 

 this conclusion. But large nuclei, large penumbraB, striae, faculas, do 

 they indicate an abundant, luminous, and calorific emission, as Herschel 

 supposed? That would be the result of his hypothesis on the exist- 

 ence of very active ascending currents, but direct experience seems to 

 contradict it. 



The following is the way in which a learned physicist, Sir David 

 Brewster, appreciates this view of Herschei's: "It is not couceirable 

 that luminous clouds, ceding to the lightest impulses and in a state of 

 constant change, can bo the source of the sun's devouring flame and of 

 the dazzling light which it emits; nor can Ave admit, besides, that the 

 feeble barrier formed by planetary clouds would shelter the objects 

 that it might cover from the destructive effects of the superior elements." 

 Sir David Brewster imagines that the non-luminous rays of caloric, 

 which form a constituent part of the solar light, are emitted by the dark 

 nucleus of the sun; wdiile the visible colored rays proceed from the 

 luminous matter by which the nucleus is surrounded, "From thence," 

 he says, " proceeds the reason of light and heat always appearing in a 

 state of combination ; the one emanation cannot be obtained without 

 the other. With this hypothesis we could readily explain why it is 

 hottest when there are most si)ots, because the heat of the nucleus would 

 then reach us without having been weakened by tlie atmosphere that it 

 usually has to traverse." But it is far from being an ascertained fact 

 that we experience increased heat during the apparition of solar spots; 

 the inverse phenomenon is more probably true. 



Herschel also studied the physical constitution of the moon. In 17S0, 

 besought to measure the height of the mountains of our satellite. The 

 conclusion that he drew from his observations was, that few of the lunar 

 mountains exceed 800 meters, (or 2,000 feet.) More recent seleno- 

 graphic observations give conclusions different from this. There is 

 reason to remark, on this occasion, how much the result presented by 

 Herschel differs from any tendency to the extraordinary or the gigantic, 

 that has been so unjustly assigned as the characteristic of the illustrious 

 astronomer. 



At the close of 1787, Herschel presented a memoir to the Eoyal So- 

 ciety, the title of which must have made a strong impression on the popu- 

 lar mind. The author therein relates that, on the 19th of April, 1787, 

 he had observed in the non-illuminated part of the moon — that is, in 



