THE PHILOSOPHY OF DEW. 



of temperature which afterwards disappeared, so that at day-break all the thermometers 

 were sensibly at the same height. 



These facts appeared to the opponents of Wells' principle completely decisive; and from 

 that time forward they maintained that " the pretended cold which is indispensable for 

 the formation of dew is a pure invention!" But the experiments of these gentlemen were 

 made near the soil, in an atmoshphere charged with moisture; all the tubes of the ther- 

 mometers were uncovered; and in the last experiment their bulbs communicated by means 

 of the plates, with the tubes supporting them. Now glass, of which these tubes were 

 made, radiates considerably; its temperature falls and the cold acquired is communicated 

 to the bodies touching it; the latter being in a moist atmosphere cause a deposit of aque- 

 ous vapor; and we know that water radiates heat and cools as much as glass, lampblack, 

 or varnish. There is then nothing surprising in the fact that the thermometers on the 

 plates marked, after a certain time, the same temperature as those surrounded with bet- 

 ter radiating substances. From the fact that the metallic surfaces covered with dew were 

 as cold as the glass or blackened surfaces, we can only conclude that the radiating powers 

 of water, lampblack and glass, are sensibly equal; but we can by no means, from these 

 experiments, say that metals cool on calm fine nights as much as glass or lampblack. 



In order to be certain of the true state of things, glass must not be employed; the sup- 

 ports must be made of polished tin, which hardly radiates at all, and which sufficiently 

 isolates the thermometers from the soil; moreover, all the parts of the thermometers must 

 be covered with metal. Then, the metal being polished, the thermometers will give the 

 true temperature of the air; and when the casing is varnished, blackened, covered with 

 leaves or any other substance, we obtain by a simple comparison with the polished ther- 

 mometer, the degree of cold produced hj the radiation of this substance. 



By means of such instruments as these, I have found that leaves of plants, glass, var- 

 nish and lampblack, always cool on calm fine nights, from one to two degrees below the 

 surrounding air. On looking at the smallness of these differences, one might be led to 

 suppose that the fall of temperature observed by Wilson and Wells, and which amounted 

 to seven or eight degrees, was much exaggerated. But when we remember that in their 

 experiments the tliermomcters for showing the temperature of the air were raised four or 

 five feet above the earth, while those covered with the radiating substance were close to 

 the soil, we can easily see why their results and my own differ so much. For Pictet has 

 long ago shown that the temperature of the air decreases rapidly, on calm fine nights, as 

 we approach the earth. This fact alone would render the temperature of the radiating 

 substance, placed close to the surface of the earth, lower than that of the air in which the 

 higher thermometers were placed; so that, in this arrangement of the instruments, the 

 difference between the two thermometers by no means indicates the amount of cooling of 

 the bod}' below the surrounding air. 



In another of Wells' experiments there was a thermometer covered with wool placed 

 at the same level as a free thermometer, and the difference of temperature observed was 

 5°, 3. Here the wool certainly cooled two or three times as much as the lampblack in my 

 experiments ; and I know that the radiating power of wool is not greater than that of 

 lampblack. 



To explain the cause of this extraordinary cold observed by Wells, we must first clear 

 up any doubt that may be attached to it. It was for this purpose that I covered a ther- 

 mometer with wool, and exposed it to the air with two others of the same size, one of 

 which was coated with lampblack and the other with polished metal; in a few minutes 

 the thermometer with the wool fell twice as low as that coated with lampblack. A fourth 



