350 
PROFESSOR TYNDALL ON THE ACTION OP FREE MOLECULES ON 
powerful refrigeration. I close these references to mist and haze by mentioning a 
most striking observation made by Wells on the 1st January, 1814. “I found,” he 
says, “ during a dense fog, while the weather was very calm, a thermometer lying on 
grass thickly covered with hoar frost, 9° lower than another suspended in the air 
4 feet above the former.” - * Here, as before, low temperature implies scanty vapour, 
the absence of which enabled the grass to pour its heat even through the interstices of 
a dense fog.t 
I could draw still further on this admirable Essay in illustration of the thesis which I 
have so long defended. As a repertory of valuable facts and penetrative arguments 
it probably 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 arrest¬ 
ing 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 ?) concluded 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 
progress from the earth towards the heavens.” J Wells had the strongest analogies 
to adduce in favour of this view. He placed boards and sheets of paper above his 
thermometers, thus screening 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 protection 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.”§ 
* Ibid., p. 158. 
f Mr. Glaishek moreover has found differences of from 1.0° to 12° between grass and air “ at times 
when the shy has been free from clouds but not bright, haze and vapour being prevalent.” (Phil. 
Trans., 1847, p. 145.) 
J Essays, p. 205. 
§ Ibid., p. 206. “ If,” says Wells, in another place, “the clouds were high and the weather calm, I 
