WILLIAMSON ON COMETS. 190 



sun, are not oppressed by the vast weight of their winter clothing. And 

 though, in the day time, they might be rather too warm, even when 

 relieved from the chief part of their ponderous atmosphere, while in 

 their perihelio, they are constantly relieved by a quick circulation of 

 air from behind the nucleus. Their day also is probably very short. 

 This inference is only drawn from analogy ; for as no spots have hither- 

 to been observed on the surface of a comet, no means have presented 

 of determining its diurnal revolutions. But of the planets, we observe 

 that those nearest to the sun have commonly the longest day. Venus, 

 at the distance of fifty-nine millions of miles from the sun, turns round 

 her axis in five hundred and eighty-four hours. Mars, at the distance 

 of one hundred and twenty-three millions of miles from the sun, revolves 

 round his axis in twenty-four hours and forty minutes ; and Jupiter, at 

 the distance of four hundred and twenty-four millions of miles from the 

 sun, revolves round his axis in nine hours and fifty-six minutes, though 

 he is a great planet. 



I have been speaking of light as a body that comes from the sun. 

 The particles, indeed, are very small ; but they have figure and size, and 

 they obviously retain their luminous appearance, while they are in mo- 

 tion, else we should not see thousands of other suns, from some of which 

 the light must be hours, weeks, months, or years in coming. But when 

 those particles fall upon this globe, or other solid body, such of them as 

 are not reflected lose, in a short time, their luminous appearance. Hence 

 it is that, in the absence of the moon, we have much more light, one 

 hour after the sun sets, than in the morning, one hour before he rises; 

 by which time the luminous appearance of the particles from the sun 

 is extinguished. It has been objected that if light be a substance 

 coming from the sun, the size of that luminary must decrease. Such 

 is doubtless the case ; nor has it been conceived that our system is to 

 be perpetual. The decrement, however, of the sun is very slow, not 



