290 HERSCHEL. 



Sir David Brewster, appreciates this view of Herschel's : 

 " It is not conceivable that luminous clouds, ceding to 

 the lightest impulses and in a state of constant change, 

 can be the source of the sun's devouring flame and of 

 the dazzling light which it emits ; nor can we 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 D. 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 ; whilst 

 the visible coloured 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 should explain naturally why it is 

 hottest when there are most spots, because the heat of 

 the nucleus would then reach us without having been 

 weakened by the atmosphere that it usually has to tra- 

 verse." 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 prob- 

 ably true. 



Herschel occupied himself also with the physical con- 

 stitution of the moon. In 1780, he sought to measure 

 the height of our satellite's mountains. The conclusion 

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

 lunar mountains exceed 800 metres (or 2600 feet). 

 More recent selenographic studies differ from this con- 

 clusion. There is reason to observe on this occasion 

 how much the result surmised by Herschel differs from 

 any tendency to the extraordinary or the gigantic, that 



