our antipodes. It need hardly be said that no spots, whose effects would be comparable with those produced by such a disk of blackness, have ever been seen upon the face of the sun. Spots are not black or nearly black, even in their very nucleus. The largest ever seen has not had an extent approaching that of our imagined black disk, even when the whole dimensions of the spot—nucleus, umbra, and penumbra—have been taken into account. Moreover, all round a spot there is always a region of increased brightness, making up to a great degree, if not altogether, for the darkness of the spot itself. So that unquestionably the summer heat in the Southern Hemisphere exceeds the summer heat in our hemisphere to a much more marked degree than the heat given out by the sun when he is without spots exceeds the heat of a spotted sun.
It is, however, rather difficult to ascertain what effect is to be ascribed to this peculiarity. It is certain that the Australian summer differs in several important respects from the European summer; but it is not easy to say how much of the difference is due to the peculiarity we have been considering, and how much to the characteristic distinction between the northern and southern halves of the earth—the great excess of water-surface over land-surface in the Southern Hemisphere. It is worthy of notice, however, that even in this case, where we cannot doubt that a great difference must exist in the solar action at particular seasons, we find ourselves quite unable to recognize any peculiarities of weather as certainly due to this difference.
I have spoken of a second way of viewing the difference in question, by considering it as it affects the whole earth. The result is sufficiently surprising. It has been shown, by the researches of Sir J. Herschel and Pouillet, that on the average our earth receives each day a supply of heat competent to heat an ocean, 260 yards deep over the whole surface of the earth, from the temperature of melting ice to the boiling-point. Now, on or about June 30th the supply is one-thirtieth less, while on or about December 30th the supply is one-thirtieth greater. Accordingly, on June 30th, the heat received in a single day would be competent only to raise an ocean 251⅓ yards deep from the freezing to the boiling point, whereas on December 30th the heat received from the sun would so heat an ocean 268⅔ yards deep. The mere excess of heat, therefore, on December 30th, as compared with June 30th, would suffice to raise an ocean, more than 17 yards deep and covering the whole earth, from the freezing-point to the temperature of boiling water! It will not be regarded as surprising if terrestrial effects of some importance should follow from so noteworthy an
tion of 31 to 30. Hence the size of Ms disk varies in the proportion of 31 times 31 to 30 times 30, or as 961 to 900. The defect of the latter number 900 amounts to 61, which is about a sixteenth part of the larger number. But a black disk having a diameter equal to a quarter of the sun's would cut off precisely a sixteenth part of his light and heat, which was the fact to be proved.
