522 Prof. Tyndall on the Action of Free Molecules on 



I could draw still further on this admirable Essay in illus- 

 tration of the thesis which I have so long defended. As a 

 repertory of valuable facts and penetrative arguments it pro- 

 bably stands unrivalled in the literature of meteorology. One 

 point remains which cannot be passed over. It has reference 

 to the part played by clouds in arresting and returning the 

 radiation from the earth. " No direct experiments," says 

 "Wells, " can be made to ascertain the manner in which clouds 

 prevent, or occasion to be small, the appearance of a cold 

 at night, upon the surface of the earth, greater than that of 

 the atmosphere; but it may, I think, be firmly [fairly?] con- 

 cluded from what has been said in the preceding article, that 

 they produce this effect, almost entirely, by radiating heat to 

 the earth, in return for that which they intercept in its pro- 

 gress from the earth towards the heavens " *. YV ells had the 

 strongest analogies to adduce in favour of this view. He placed 

 boards and sheets of paper above his thermometer, thus screen- 

 ing them from the clear sky; and in that beautiful passage 

 where he speaks of " the pride of self-knowledge," and refers 

 to the simple devices which experience had taught gardeners 

 to apply for the safety of their plants, he mentions the pro- 

 tection which even a thin cambric handkerchief can afford to 

 thermometers over which it is spread. He was irresistibly 

 led to conclude that clouds acted in the same fashion, and that 

 when they occupied the firmament, they sent back to the earth 

 the heat incident upon them, exactly as the board, and the 

 paper, and the cambric sent it back in experiments made close 

 to the surface of the earth. 



But in the enunciation of this hypothesis his knowledge and 

 penetration as an observer came, as usual, into play. He is 

 careful to distinguish between high clouds and low clouds. 

 "Dense clouds," he says, "near the earth must possess the 

 heat of the lower atmosphere, and will therefore send to the 

 earth as much, or nearly as much, heat as they receive from 

 it by radiation. But similarly dense clouds, if very high, 

 though they equally intercept the communication of the earth 

 with the sky, yet being from this elevated situation colder 

 than the earth, will radiate to it less heat than they receive 

 from it, and may consequently admit of bodies on its surface 

 becoming several degrees colder than the air "|- 



Magnus urged this point against me; and I may be per- 



* Essays, p. 205. 



t Ibid. p. 206. " If," says Wells, in another place, " the clouds 

 were high and the weather calm, I have sometimes seen on grass, though 

 the sky was entirely hidden, no very inconsiderable quantity of dew " 

 (ibid. p. 128), 





