Derstanding; but on looking over several periodicals I think that it has been entirely misrepresented. 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 precisely 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 1°,5 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 thermometers always indicates the coolest temperatures, we conclude that the differences observed between the indications of the two pairs of instruments, arise solely from the different temperatures of the atmospheric layers in which they are placed; and that consequently, on calm and clear nights, the temperature of the air decreases rapidly as we approach the earth.

Now the experiment on which Wells' theory rests, consists in the often-repeated observation 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 medium 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 because 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 played 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 inability to cool by radiation, the air close to the earth contributes powerfully to lower the temperature of the plants in it, by means of a series of actions and reactions, the causes and effects of which have, if I am not mistaken, been clearly defined in the second of the two letters which form the object of this discussion. Those who have clearly seen their true meaning will permit me, without doubt, to save them the annoyance of a useless repetition, and to refer the editors of the papers of which I spoke above, to a rather more attentive perusal of the numbers of the Comptes rendus, where they are inserted. After which they will be quite at liberty to show that I am wrong; but they will first admit, I hope, that they have misinformed their readers with respect to the consequences resulting from my work on the phenomena of dew.