486 THE POPULAR SCIENCE MONTHLY. 



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 penum- 

 bra have been taken into account. Moreover, all round a spot there 

 is always a region of increased brightness, making up to a great de- 

 gree, 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 peculiar- 

 ity we have been considering, and how much to the characteristic dis- 

 tinction between the northern and southern halves of the earth the 

 great excess of water-surface over land-surface in the Southern Hemi- 

 sphere. 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 ques- 

 tion, by considering it as it affects the whole earth. The result is suf- 

 ficiently 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-thir- 

 tieth 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 251^ yards deep from 

 the freezing to the boiling point, whereas on December 30th the heat 

 received from the sun would so heat an ocean 268f yards deep. The 

 mere excess of heat, therefore, on December 30th, as compared with 

 June 30th, would suffice to raise an ocean, more than lY yards deep 

 and covering the whole earth, from the freezing-point to the temper- 

 ature of boiling water ! It will not be regarded as surj^rising if terres- 

 trial 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 U i larger number. But a black disk having a diame- 

 ter equal to a quarter of the sun's wo ild cut off precisely a sixteenth part of his light 

 and heat, which was the fact to be proved. 



