When we consider the case of heat accompanied by light, we understand readily enough that a screen may interfere with the emission of radiant heat. We use a fire-screen, for instance, with the object of producing just such an interference. But we are apt to forget that what is true of luminous heat is true also of that heat which every substance possesses. In fact, we do not meet with many instances in which the effect of screens in preventing the loss of obscure heat is very noteworthy. There are some, as the warmth of a green-house at night, and so on; but they pass unnoticed, or are misunderstood. It was in this way that the explanation of dew-phenomena had been so long delayed. The very law on which it is founded had been practically applied, while its meaning had not been recognized. “I had often in the pride of half-knowledge,” says Wells, “smiled at the means frequently employed by gardeners to protect tender plants from cold, as it appeared to me impossible that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But when I had seen that bodies on the surface of the earth become, during a still and serene night, colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice which I had before deemed useless.”
And now all the facts which had before seemed obscure were accounted for. It had been noticed that metallic plates were often dry when grass or wood was copiously moistened. Now, we know that metals part unwillingly with their heat by radiation, and therefore the temperature of a metal plate exposed in the open air is considerably higher than that of a neighbouring piece of wood. For a similar reason, dew is more freely deposited on grass than on gravel. Glass, again, is a good radiator, so that dew is freely deposited on glass objects,—a circumstance which is very annoying to the telescopist. The remedy employed is founded on Wells’ observations—a cylinder of tin or card, called a dew-cap, is made to project beyond the glass, and thus to act as a screen, and prevent radiation.
We can now also interpret the effects of a clear sky. Clouds act the part of screens, and check the emission of radiant heat from the earth. This fact has been noticed before, but misinterpreted, by Gilbert White of Selborne. “I have often observed,” he says, “that cold seems to descend from above; for when a thermometer hangs abroad on a frosty night, the intervention of a cloud shall immediately raise the mercury ten degrees, and a clear sky shall again compel it to descend to its former gauge.” Another singular mistake had been made with reference to the power which clouds possess of checking the emission of radiant heat. It had been observed that on moonlit nights the eyes are apt to suffer in a peculiar way, which has occasionally brought on temporary blindness. This had been ascribed to the moon’s influence, and the term moon-blindness had therefore been given to the affection. In reality, the moon has no more to do with this form of blindness than the stars have to do with the formation of dew. The absence of clouds from the air is the true cause of the mischief. There is no sufficient check to the radiation of heat from the eyeballs, and the consequent chill results in temporary loss of sight, and sometimes even in permanent injury.
Since clouds possess this important power, it is clear that while they are present in the air there can never be a copious formation of dew, which requires, as we have seen, a considerable fall in the temperature of the air around the place of deposition. When the air is clear, however, radiation proceeds rapidly, and therefore dew is freely formed.
But it might seem that since objects in the upper regions of the air part with their radiant heat more freely than objects on the ground, the former should be more copiously moistened with dew than the latter. That the fact is exactly the reverse is thus explained. The cold which is produced by the radiation of heat from objects high in the air is communicated to the surrounding air, which, growing heavier, descends towards the ground, its place being supplied by warmer air. Thus the object is prevented from reducing the air in its immediate neighbourhood to so low a temperature as would be attained if this process of circulation were checked. Hence, a concave vessel placed below an object high in air, would serve to increase the deposition of dew by preventing the transfer of the refrigerated air. We are not aware that the experiment has ever been tried, but undoubtedly it would have the effect we have described. An object on the ground grows cold more rapidly, because the neighbouring air cannot descend after being chilled, but continues in contact with the object; also cold air is continually descending from the neighbourhood of objects higher in air which are parting with their radiant heat, and the cold air thus descending takes the place of warmer air, whose neighbourhood might otherwise tend to check the loss of heat in objects on the ground.
Here, also, we recognize the cause of the second peculiarity detected by Aristotle—namely, that dew is only formed copiously in serene weather. When there is wind, it is impossible that the refrigerated air around an object which is parting with its radiant heat, can remain long in contact with the object. Fresh air is continually supplying the place of the refrigerated air, and thus the object is prevented from growing so cold as it otherwise would.
In conclusion, we should wish to point out the important preservative influence exercised during the formation of dew. If the heat which is radiated from the earth, or from objects upon it, during a clear night, were not repaired in any way, the most serious injury would result to vegetation. For instance, if the sun raised no vapour during the day, so that when night came on the air was perfectly dry, and thus the radiant heat passed away into celestial space without compensation, not a single form of vegetation could retain its life during the bitter cold which would result. But consider what happens. The sun’s heat, which has been partly used up during the day in supplying the air with aqueous vapour, is gradually given out as this vapour returns to the form of water. Thus the process of refrigeration is effectually checked, and vegetation is saved from destruction. There is something very beautiful in this. During the day, the sun seems to pour forth his heat with reckless profusion, yet all the while it is being silently stored up; during the night, again, the earth seems to be radiating her heat too rapidly into space, yet all the while a process is going on by which the loss of heat is adequately compensated. Every particle of dew which we brush from the blades of grass, as we take our morning rambles, is an evidence of the preservative action of nature.