54 



KNOWLEDGE. 



[March 1, 1892. 



otlier in a longer, in proportion to their unequal area : but 

 the gi-eater proximity of the sun in the smaller segment com- 

 pensates exactly for its more rapid description, and thus an 

 equilibrium of heat is. as it were, maintained. Were it not 

 for this, the excentricity of the orbit would materially 

 influence the transition of seasons. The fluctuation of 

 distance amounts to nearly J^fth of its mean quantity, 

 and, consequently, the fluctuation in the sun's direct 

 heating power to double this, or j'jth of the whole. 

 Now, the perihelion of the orbit is situated nearly at the 

 place of the northern winter solstice ; so that, were it not 

 for the compensation we have just described, the effect 

 would be to exaggerate the difference of summer and 

 winter in the southern hemisphere, and to moderate it in 

 the northern ; thus producing a more violent alternation 

 of climate in the one hemisphere, and an approach to 

 perpetual spring in the other. As it is, however, no such 

 inequality subsists, but an equal and impartial distribution 

 of heat and light is accorded to both." 



In the fifth edition, published in 1858, this Article was 

 replaced by the three following : — 



'• (868 ».) Let us next consider how these phft-nomena are 

 modified by the ellipticity of the earth's orbit and the 

 position of its longer axis with respect to the hue of the 

 solstices. This ellipticity (art, 850) is about one sixtieth of 

 the mean distance, so that the sun, at its greatest proximity 

 is about one thirtieth of its mean distance nearer us than 

 when most remote. Since light and heat are equally dispersed 

 from the sun in all directions, and are spread, in diverging, 

 over the surface of a sjihere enlarging as they recede from the 

 center, they must diminish in intensity according to the in- 

 verse proportion of the surfaces over which they are spread, 

 /.(". in the inverse ratio of the squares of the distances. Hence 

 the hemisphere opposed to the sun will receive in a given 

 time, when nearest, two thirtieths or one fifteenth more 

 heat and light than when most remote, as may be shown by 

 an easy calculation. Now, the sun's longitude when at its 

 least distance from the earth (at which time it is said to be 

 in perigee and the earth in its perihelion) is at present 280'^ 

 28' in which position it is on tlie 1st of January, or eleven 

 days after the time of the winter solstice of the northern 

 hemisphere ; or, which is the same thing, the summer 

 solstice of the southern (art. 864), while on the other hand 

 the sun is most remote (in apogee or the earth in its 

 aphelion), when in longitude 100° 28' or on the 2nd of July, 

 /.(•. eleven days after the epoch of the northern summer or 

 southern winter solstice. We shall suppose, however, for 

 simplicity of explanation, the perigee and apogee to be 

 coincident with the solstice. At and about the southern 

 summer solstice then, the whole earth is receiving per dieiii 

 the greatest amount of heat that it can receive, and of this 

 the southern hemisphere receives the larger share, because 

 its pole and the whole region within the antarctic circle is 

 in perpetual sunshine, while the corresponding northern 

 regions lie in shadow. On the other hand, at and about the 

 northern summer solstice, although it is true that the reverse 

 conditions as to the regions illuminated prevail, yet the 

 whole earth is then receiving jjcr dirm less heat owing to 

 the sun's remoteness : so that on the whole //' tin- sctitious 

 irere of equal dwettion, or in other words, if the angular 

 movement of the earth in its elliptic orbit were uniform, 

 the southern hemisphere would receive more heat per 

 annum than the northern, and would consequently have a 

 warmer mean temperature. 



" (868 h.) Such, however, is not the case. The amjidar 

 velocity of the earth in its orbit, as we have seen (art. 350), 

 is not uniform, but varies m the inverse ratio of the square of 

 the sun's distance, that is, in the same precise ratio as his 

 heating power. The momentary supply of heat then 



received by the earth in every point of its orbit varies 

 exactly as the momentary increase of its longitude, from 

 which it obviously foUows, that equal amounts of heat are 

 received from the sun in passing over equal angles round 

 it, in whatever part of the ellipse those angles may be 

 situated. Supposing the orbit, then, to be divided into two 

 segments by any straight line drawn through the sun, 

 since equal angles in longitude (180^) are described on 

 either side of this line, the amount of heat received will 

 be equal. In passing then from either equinox to the other, 

 the whole earth receives equal amount of heat, the inequality 

 in the intensities of solar radiation in the two intervals 

 being precisely compensated by the opposite inequality in 

 the duration of the intervals themselves ; which amounts 

 to about 7 3 days, by which the northern spring and summer 

 are together longer than the southern. For these intervals 

 are to each other in the proportion of the two imequal 

 segments of the whole ellipse into which the Une of the 

 equinoxes divides it (see art. 353j. 



" (868 c.) In what regards the comfort of a cUmate and 

 the character of its vegetation, the intensity of a summer 

 is more naturally estimated by the temperature of its hottest 

 day, and that of a winter by its sharpest frosts, than by the 

 mere durations of those seasons and their total amount of 

 heat. Supposmg the excentricity of the earth's orbit were 

 very much greater than it actually is ; the position of its 

 perihelion remaining the same ; it is evident that the 

 characters of the seasons in the two hemispheres would be 

 strongly contrasted. In the northern, we should have a 

 short but very mild winter, with a long but very cool 

 summer — i.e. an approach to perpetual spring ; while the 

 southern hemisphere would be inconvenienced and might 

 be rendered uninhabitable by the fierce extremes caused by 

 concentrating half the annual supply of heat into a summer 

 of very short duration and spreading the other half over a 

 long and dreary ^^•inter, sharpened to an intolerable 

 intensity of fi'ost when at its climax by the much greater 

 remoteness of the sun." 



Now, it is quite clear that Sir Eobert Ball's suggestion 

 attributing the origin of the mis-statement to Sir J. 

 Herschel's having hastily confused two very distinct 

 things, can by no means have been correct ; because the 

 remarkable proposition mentioned on page 120 of The 

 ( 'ause I if an Ice A(je was stated very clearly in the 

 paragraph of the Outlines next preceding that which 

 Sir E. Ball incriminates ; and there Sir John Herschel 

 was dealing with actual fact, whereas in the subsequent 

 paragraph he was considering a supposititious case. 



Surely it is more reasonable to suppose that he used the 

 word luilf (with extraordinary lack of his usual carefulness), 

 instead of part, to import mere division, without regard to 

 quantity. This use of the word cannot possibly be defended, 

 but the most careful writers are liable to similar slips ; and 

 it seems inconceivable that he could have meant to represent 

 the heat received by either hemisphere during the year as 

 equally divided between summer and winter, imder any 

 circumstances whatever. He includes heat and light as 

 subject to the same law of difl'usion, and who could possibly 

 think or say that the summer and winter light could ever 

 be equal "? 



We need not look far for inaccuracies somewhat analo- 

 gous. The quotation from Sir Eobert Ball, first above cited, 

 reads as if he accused Herschel of formally teaching the 

 aUeged equality to be the normal or constant state of 

 matters, but his indictment can hardly be meant to go so 

 far as that. His book speaks of the mistake as an 

 inadvertence (page 185). 



Again, on page 27 of Tlie Cause nf an Ice Aye, he says, 

 " We have learned that one hemisphere was once covered 



