Hence we see, that in order that a body may be covered by dew it must first cool to a certain degree, and the reasons why metals do not become covered with dew, is that they do not become sufficiently cold by radiation. But is there nothing else to hinder the precipitation of dew on metals? In other words, is the feeble radiating power of metals the true and only reason why they are never wetted by dew?

The following experiment seems to me to answer this question, at the same time that it refutes the theories that dew rises from the earth or falls from the clouds, and clearly proves Well's principle: On a tin disc, as large and thin as possible, draw a concentric circle with a radius equal to one-third of that of the disc, and cover it with a thick layer of varnish. Then take another tin disc less by ten millimetres than the varnished circle; and having soldered a pointed iron wire (2 millimetres large and 2 or 3 decimetres long,) at its centre, and perpendicular to it, place the wire through a hole in the center of the great disc on the varnished side. The great disc is to be pushed along the wire until the two discs arc about five millimetres from each other, at which distance they must be kept.

The discs so joined are to be taken in the evening out in the fields, and left for a few minutes in a horizontal position, quite out of contact with any other bodies. If the night is calm and fine, phenomena which one might easily foretell will soon be observed on the surface of the larger disc.

It suffices, indeed, to remember that in the position in which the instrument was left, the small disc was uppermost, and hence an annular band of the varnished part of the lower disc will be exposed to the air. Now it is clear that this band will radiate heat, cool, become covered with dew, and will propel cold and dew in consequence, from the side next the center, and from that next the circumference. This propagation will, however, whilst the varnish beneath the small disc can only cool by contact. And we find that the central part of the varnished circle remains perfectly dry, whereas the outer metallic band is wetted to its very edge, if the atmosphere is exceedingly damp.

But there is another circumstance which we could not so easily have foreseen. I mean the exact repetition of the same appearances on the surface of the disc turned next the ground. Dew begins to appear on this surface exactly opposite the little exterior varnished band; a light whitish circle suddenly appears on the polished metal, and after becoming better defined, spreads little by little, sometimes as far as the edges of the plate in one direction, but never reaches the center, which remains quite dry and brilliant like the corresponding portion on the other side of the disc, and the small circular roof which covers it without touching it.

From this very simple experiment we draw the following conclusions. Dew does not fall from the sky, because the upper disc is always dry, and the greater part of the lower one is wetted. It does not arise from the ground, because if the exterior part of the lower surface of the great disc is covered, the middle part of it remains perfectly dry and brilliant. Metals do not repel aqueous vapor which forms dew, nor do they cause its evaporation, as it is deposited upon them, since we see some parts of the metal quite wet, and others quite dry.

The appearance of the dew first on the uncovered band of varnish, and its gradual extension to the other adjacent and opposite parts of the great disc, together with the fall of temperature observed on the varnished cases of the thermometers exposed freely to the air, prove that dew is a pure consequence of nocturnal radiation, which gives to good radiators the degree of cold necessary for the condensation of the elastic invisible aqueous vapor which pervades our atmosphere.

Thus far all the facts agree perfectly with the generally received theory of dew. In a second letter I shall bring forward others which are not capable of explanation in the usual way, but which, nevertheless, are connected in a fortunate manner to the principle of Wells.

Second Letter

In this letter I shall show, as I before promised, that although Wells' principle of the origin of dew is correct, it is impossible to explain all the thermometrical and hygromet-rical phenomena which are produced during a calm fine night, without taking into account a new circumstance, as yet entirely neglected, which is of the greatest consequence in the nocturnal cooling of bodies. But before proceeding to this point, I shall offer a few remarks on two series of experiments, advanced by those who maintain that dew arises from the soil, and which they say completely refute all theories of dew based upon radiation.

A certain number of thermometers, as nearly as possible equal, both in size or sensibility, arc taken, of which some are coated with lampblack, varnish and Indian ink, while others are covered with gold, silver, tin, copper, and leaves of plants. These instruments, thus prepared and exposed to the free atmosphere on a calm fine night, indicated at first temperatures differing a little the one from the other; but after a certain space of time, they all indicated very nearly the same temperature. The experiment was varied - on the ends of glass tubes placed upright in the soil, plates of zinc, copper, glass, were laid; in the center of each of these plates was a depression in which the bulb of the thermometer was placed, while its stem, supported by an iron wire, remained in a vertical position; another thermometer suspended freely between the plates marked the temperature of the air. Here again the instruments, when exposed to the air in the evening, indicated differences, 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 far ward they maintained that "the pretended cold which is indispensable for the formation of dew is a pure invention!" Bat the experiments of these gentlemen were made near the soil, in an atmoshphere charged with moisture; all the tubes of the thermometers 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 aqueous 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 better 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.