THE PHILOSOPHY OP DEW. 



derstanding; but on looking over several periodicals I think that it has been entirely mis- 

 represented. In short, the editors of these papers, having perhaps the same opinion as 

 he who first among them noticed my theory, cite the first part of it, and pass over the 

 second in silence; the reader is hence led to believe that my work tends only to confirm 

 Wells' theory as explained in all treatises on physics and meteorology; while it is precise- 

 ly to an opposite conclusion that my experiments lead. I shall try to render myself more 

 intelligible by setting out from those data on which the theory is based. 



Take two pairs of thermometers enveloped in their metallic case, and hung, by means 

 of metallic threads, in the manner described in my first letter. Suppose that each of these 

 pairs is composed of one thermometer with a polished, and another with a blackened case. 

 Let us suppose lastly, that, on a calm and clear night, one of these pairs be fixed close to 

 the surface of an exposed meadow, while the other be placed four or five feet above the 

 surface, so that the two thermometers of each pair are at the same level. 



After a short exposure, the black thermometers will be seen to descend about l^jS 

 below the metallic thermometers beside them. However, the temperatures indicated by 

 the lower will be very different from those marked by the upper pair; the difference will 

 amount to five or six degrees, if the night is calm and fine; and as the lower pair of ther- 

 mometers always indicates the coolest temperatures, Ave conclude that the differences ob- 

 served between the indications of the two pairs of instruments, arise solely from the dif- 

 ferent temperatures of the atmospheric layers in which they are placed; and that conse- 

 quently, on calm and clear nights, the temperature of the air decreases rapidly as we ap- 

 proach the earth. 



Now the experiment on which Wells' theory rests, consists in the often-repeated obser- 

 vation that a common thermometer, placed in contact with the grass, indicates a much 

 lower temperature than a thermometer raised four or five feet above the soil; whence it 

 has been concluded that the grass is cooled many degrees by radiation towards the sky. 

 * * * But it is easy to convince yourself that this conclusion is quite unauthorised. 

 In short, place one of your uncovered thermometers in contact with the grass, and let the 

 other hang freely in the air, at the same distance from the ground, you will find that the 

 two instruments mark the same degree. Now nobody would deny that this is the way 

 to proceed to show, according to the old method, the cooling of the grass below the medi- 

 um surrounding it. We are forced, then, to conclude that the fundamental data of Wells' 

 theory are inconclusive — 1st, because the surfaces of the thermometers employed radiated 

 quite as much as the blades of grass; 2d, because the thermometer destined to measure 

 the temperature of the air was placed in an atmospheric layer much warmer than that 

 which surrounds the grass in contact with the other thermometer. 



The principle that the deposition of dew is owing to the cold caused by radiation, is, I 

 repeat, perfectly just, but Wells' theory is incorrect. The reason of this is evidently be- 

 cause the influence of the air in the production of the cold which is continually developed 

 near the surface of the earth, has been entirely neglected. It has been vaguely said, it is 

 true, that radiating bodies, placed at a certain height, do not lower in temperature so 

 much as those placed close to the ground, in consequence of descending currents which 

 are formed around the first, and are absent from the second. But that was insufficient to 

 show the true part plaj^'cd by the air in the formation of dew. 



It was necessary to prove, as I think I was the first to do, that notwithstanding its in- 

 abilitj^ to cool b}' radiation, the air close to the earth contributes powerfully to lower the 

 ature of the plants in it, by means of a series of actions and reactions, the causes 

 ffects of which have, if I am not mistaken, been clearly defined in the second of the 



