24 REMARKS ON SIR ROBERT BALL's PAPER. 



5th edition of Herschel's " Outlines of Astronomy," which 

 runs thus : — 



" Supposing the eccentricity of the earth's orbit were very 

 much greater than it actually is, the position of its peri- 

 helion remaining the same, it is evident that the character 

 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, — that is, 

 an approach to perpetual spring ; while the Southern Hemi- 

 sphere would be inconvenienced and might be rendered unin- 

 habitable by the fierce extremes caused by concentrating half 

 tJie annual supply of heat into a summer of very short 

 duration, and spreading the other half over a, long and dreary 

 winter, sharpened into an intolerable intensity of frost when, 

 at its climax by the much greater remoteness of the sun.'* 

 (Mark, " the annual supply," not its, etc.) 



It must be admitted that, on a cursory reading, and taking 

 it apart from its context, the above passage would appear to 

 bear only the construction which Sir Robert Ball puts upon it. 

 But is it fair, so to take it, or to take any man's utterances? 

 Herschel has just been taking great pains to make clear the 

 fact that (dividing the year and the orbit each into its — at 

 present — two unequal portions by the equinoctial diameter of 

 the orbit), though the earth is nearer the sun during the 

 .(northern) winter portion of the year than during the 

 summer 23ortion, and is consequently receiving a greater 

 intensity of heat (according to the law of inverse squares of 

 the distance), the earth's angular velocity being in the same 

 ratio, the shortness of the season exactly compensates the 

 intensity of radiation. He sums up the case thus : — 



" The momentary supply of heat received by the earth in 

 every jDoint of its orbit varies exactly as the momentary 

 increase of its longitude, from which it obviously follows 

 that equal amounts of heat are received from the sun in 

 whatever part of the ellipse those angles are situated. Sup- 

 posing the orbit, then, to be divided into two segments by any 

 straight line drawn through the sun, since equal angles in 

 longitude (180 deg ) 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," etc. 



Now take this quotation in connection with that to which Sir 

 Kobert Ball takes exception, as quoted above, and I think 

 Herschel's meaning will be apparent, although in this case it 

 must be admitted that he has failed to express himself with his 

 accustomed preciseness. His statement is that " the hemi- 

 sphere would be inconvenienced . . by concentrating half 

 the " (not its) " annual supply of heat into a summer of 

 short duration," etc., which is of course apportioned between. 



