THE PHILOSOPHY OF DEW. 



thermometer covered with au equal quantity of wool, pressed close to the instrument by 

 means of a metallic wire, gave a result intermediate between the other two. Lastly, I 

 covered a fifth thermometer with two pieces of flannel, and it fell still less than the last. 

 These experiments were repeated, cotton being used instead of wool, and the results were 

 perfectly simihir. I then began to suspect that the superiority of the cotton and wool 

 over the lampblack, was owing to a certain modification in the radiating power of these 

 bodies, caused by the presence of the air filling their interstices. 



But how can air increase the cold resulting from radiation.'' The answer is simple. We 

 have known for many years, that the nocturnal cooling of a body does not vary with the 

 temperature of the atmosphere. Thus Capts. Parry and Scoresby state that on calm fine 

 nights in the polar regions, the snow was cooled about 9'^ below the air four or five feet 

 above it when the temperature of the latter was 0*^ — or 25° — or SO'^. M. Pouillet has 

 found that Swan's down is cooled 7° below the air at 0° or — 25''. And I for my 

 part, have found that the blackened or varnished thermometers cool a certain fixed num- 

 ber of degrees, whatever the temperature of the night may be. Now it is clear that 

 the tufts of cotton or of wool spread out on the upper part of the bulb of a thermome- 

 ter, after having cooled by radiation, will communicate the cold so acquired to the sur- 

 rounding air, which becomes by this means heavier, will descend in the interior to fall on 

 the ground ; but a certain time is required for the passage of this air through the inter- 

 stices of the wool or cotton. The threads then, of these last, will be in contact with air 

 that is colder than it was at the beginning of the experiment; and as the fall in this tem- 

 perature below the surrounding medium is invariable, they must necessarily become cold- 

 er still. This increase of cold will cause a new fall of temperature in the medium; the 

 latter gives rise to another cooling in the radiating body; and so on until the weight ac- 

 quired by the condensed air is sufiicent to overcome the obstacles opposing its exit. 



The same phenomena take place naturally in many circumstances. Indeed, plants with 

 hairjr leaves are colder than those with smooth ones. The temperature of grass and that 

 of other low plants which clothe the fields, falls, in consequence of this reaction of the air, 

 much below that of elevated bodies, because of their vicinity to the soil which supports 

 the surrounding medium, and compels it to remain in contact with the radiating surfaces. 

 The truth is, that the laj^er of air by which the grass is surrounded, is not steady; it chan- 

 ges its position, on the contrary, in precisely the same way as water in a vessel over the 

 fire; the particles of air condensed by the cold on the tops of the blades of grass, descend 

 towards the earth, become heated by contact with the latter, and rise again towards the 

 tops of the leaves, and so on; but it is clear that, in spite of this state of motion, the air 

 on the whole cools, and in order that the grass may be of the same constant temperature 

 below that of the surrounding medium, it must cool more still; and thus a gradual cool- 

 ing and an increasing moisture in the layer of air are caused. 



I cannot enter here into all the necessary details to show how the frigorific reaction of 

 the air explains all the facts preceding and accompanying the appearance of dew, and 

 many other phenomena connected with this interesting question, which have not as yet 

 been satisfactorily accounted for. They will all be found, however, in my memoir, which 

 I shall soon, I hope, have the honor to present to the academy. 



THIRD LETTER. 



My studies on dew seem to me to have proved conclusively, that if Wells' principle is 

 the theory known by the same name is erroneous, or at least exceedingly incomplete, 

 ght that I had stated this proposition so clearly as to leave no room for any misun- 



