Sir William Herschel, reasoning from terrestrial analogies, was led to look on the spot-cavities as apertures through a double layer of clouds. He argued that, were the solar photosphere of any other nature, it would be past comprehension that vast openings should form in it, to remain open for months before they close up again. Whether we consider the enormous rapidity with which the spots form and with which their figure changes, or the length of time that many of them remain visible, we find ourselves alike perplexed, unless we assume that the solar photosphere resembles a bed of clouds. Through a stratum of terrestrial clouds openings may be formed by atmospheric disturbances, but while undisturbed the clouds will retain any form once impressed upon them, for a length of time corresponding to the weeks and months during which the solar spots endure. And because the solar spots present two distinct varieties of light, the faint penumbra and the dark umbra or nucleus, Herschel saw the necessity of assuming that there are two beds of clouds, the outer self-luminous and constituting the true solar photosphere, the inner reflecting the light received from the outer layer, and so shielding the real surface of the sun from the intense light and heat which it would otherwise receive. But while recent discoveries have confirmed Sir William Herschel’s theory about the solar cloud-envelopes, they have by no means given countenance to his view that the body of the sun may possibly be cool. The darkness of the nucleus of a spot is found, on the contrary, to give proof that in that neighborhood the sun is hotter, because it parts less readily with its heat. We shall see presently how this is. Meantime let it be noticed, in passing, that a close scrutiny of large solar spots has revealed the existence of an intensely black spot in the midst of the umbra. This black spot must be regarded as the true nucleus. The circumstance that the spots appear only on two bands of the sun’s globe, corresponding to the sub-tropical zones on our own earth, led the younger Herschel to conclusions as important as those which his father had formed. He reasoned, like his father, from terrestrial analogies. On our own earth the sub-tropical zones are the regions where the great cyclonic storms have their birth, and rage with their chief fury. Here, therefore, we have the analogue of the solar spots, if only we can show reason for believing that any causes resembling those which generate the terrestrial cyclone operate upon those regions of the sun where the solar spots make their appearance.