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the descent of dew; and he accounts for it in the following manner: when there are no clouds in the air, the heat of the inferior air and of that which rises from the earth, is dissipated into the superior regions; and then the vapours, which are dispersed throughout the air, condense, and fall down in dew. But when the clouds continue, they separate the inferior from the superior part of the atmosphere, and thus prevent the dissipation of the heat, by which means the vapours remain suspended. When the sky grows cloudy, some hours after sunset, although the heat has been sensibly diminished; it is again increased; because, continuing to rise out of the earth, it is accumulated in the inferior air.-But neither can this be deemed a positive proof of the descent of dew; since we may as well suppose the heat of the atmosphere to be great enough to dissipate it in its ascent, as to keep it suspended after its ascent through the day.

On the other hand, its being found in greater quantities on bodies placed low down, than on such as are high up, is no proof of the ascent of dew; for the same thing is observed of rain. A body placed low down receives more rain than one placed in an elevated situation; and yet the rain certainly descends from the atmosphere. The reason why the dew appears first on the lower parts of bodies may be, that, in the evening, the lower part of the atmosphere is first cooled, and consequently most disposed to part with its vapour. It is also certain, that part of the water contained in the air may be condensed at any time on the sides of a glass, by means of cold, so as to run down its sides in small drops like dew. It seems, therefore, that this subject is not sufficiently determined by such experiments as have yet been made; nor indeed does appear easy to make such experiments, as shall be perfectly decisive on the matter.

But however this may be determined, it is certain that several substances, exposed to the same dew, receive and charge themselves with it in a very different manner; some more, others less, and some not at all. The drops seem to make a sort of choice of what bodies they shall fix themselves to: glass and crystals are those to which they adhere in the most ready manner, and in the largest quantity: but metals of all kinds never receive them at all, nor do the drops ever adhere to them. The reason of this is, probably, because metals promote evaporation more than glass does. Thus, if a piece of metal and a piece of glass are both made equally moist, the former will be found to dry in much less time than the latter. Hence it would seem, that there is between metals and water some kind of repulsion: and this may be sufficient to keep off the very small quantity that falls in dew; for whatever tends to make water evaporate, after it is actually in contact with any substance, tends also to keep the water from ever coming into contact with it.

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Of the dew of plants, it has been observed, that it is probably only the moisture that continually exhales through the orifices of their vessels, and not a vapour collected by their leaves; this dew being exhaled by each plant, according to the peculiar structure of its vessels, and the situation of their orifices. Indeed, it is certain, that plants, which have been shut up in vessels, and covered by: glasses, have collected a greater quantity of dew, than those that stood in the open air; and further, the drops are gathered in those parts where the orifices of the vessels are manifestly open, and not all over the leaves, nor in the upper or lower leaves only (which would be the case if they arose from ambient vapour) but in all the leaves without distinction. The oil, or honey-dew, which is sometimes exhaled in summer by trees and herbs,

and which has been found to settle on the oak, ash, &c. is of this kind.

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Mr. Thomas Melvill, a very ingenious writer in the Essays, Physical and Literary, by the Edinburgh Society, published some observations on the silver-like appearance of drops of water on the leaves of colewort. It is common (says he) to admire the volubility and lustre of drops of rain that lie on the leaves of colewort, and some other vegetables; but no philosopher, as far as I know, has put himself to the trouble of explaining this curious phenomenon. Upon inspecting them narrowly, I find, that the lustre of the drop arises from a copious reflection of light from the flattened part of its surface contiguous to the plant: I observe further, that when the drop rolls along a part which has been wetted, it immediately loses all its lustre; the green plant being then seen clearly through it: whereas, in the other case, it is hardly to be discerned.

'From these two observations laid together, we may observe, that the drop does not really touch the plant when it has the mercurial appearance, but hangs in the air at some distance from it, by the force of a repulsive power; for there could not be any copious reflection of white light from its under surface, unless there were a real interval between it and the surface of the plant.

'If that surface were perfectly smooth, the under surface of the drop would be so likewise; and would therefore show an image of the illuminating body by reflection, like a piece of polished silver: but as it is considerably rough and unequal, the under surface becomes rough likewise; and so by reflecting the light copiously in different directions, assumes the resplendent white colour of unpolished silver.

After it is proved by optical argument, that

the drop is not really in contact with the plant that supports it, we may easily conceive whence its wonderful volubility arises, and why it leaves no track of moisture where it rests.'

Mr. Melvill's explanation will enable us to account for that common phenomenon, the suspension of the drops of dew on the very summits of the blades of grass, &c. and their assuming that pearly, or resplendent white colour, which he observed in the drops on the leaves of colewort. This ingenious explanation, at the same time, shows the extensive utility of optical principles, in leading to the knowledge of things which would be otherwise inaccessible.

The limits of this paper will not allow me to enlarge on the great benefit of dews in the refreshment of the earth, and the nourishment of plants. 'How sensibly (says a pious author) has this dew refreshed the vegetable kingdom! The fervent heat of yesterday's sun, had almost parched the face, and exhausted the sweets of nature.

But what a sovereign restorative are these cooling distillations of the night! How they gladden and invigorate the languishing herbs! Sprinkled with these reviving drops, their verdure deepens, their bloom is new-flushed; their fragrance, faint or intermitted, becomes potent and copious.'

In hot countries, and in places where it rains but seldom, the dew of the night supplies, in some measure, the want of rain. In the sacred scriptures, therefore, it is represented as a great blessing: Blessed of the Lord (says Moses, speaking of Joseph) be his land for the precious things of heaven, for the dew, &c. And the want of it is represented as a curse: Ye mountains of Gilboa (said David) let there be no dew! The favour of the Divine Being is compared to the dew: "I will be (says the Lord, by Hosea) as the dew unto

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Israel: he shall grow as the lily, and cast forth his roots as Lebanon.' Heavenly doctrine, or the word of God, is likewise compared to dew: My doctrine (says Moses) shall drop as the rain, my speech shall distil as the dew, as the small rain upon the tender herb, and as the showers on the grass :' that is, my doctrine shall have the same effect upon your hearts, as dew has upon the earth: it shall render them soft, pliable, and fruitful.-But the admirable allusions to dew, in holy writ, are too numerous to be quoted.

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In a word, these transparent beauties of the morn' not only furnish us with poetic images and philosophic knowledge, but with very powerful motives also for a life of piety, benevolence, and virtue. Their great utility to the vegetable kingdom, in particular, should lead us to the unceasing adoration of that gracious Being, who created nothing, which has existence, merely for an object of idle speculation. When we consider, moreover, how silently and insensibly the light-footed dews' fulfil the gracious purposes of our common Benefactor; incessantly dispensing nutriment to vegetable life, and refreshment to animated being; how persuasively should this instruct the rich and opulent, to let their secret bounty, unostentatious as the dew of heaven, descend continually to revive the drooping hearts of modest and unassuming worth. For what are the high and mighty of the earth, without that bliss-diffusing spirit of humanity, whose exertions, while they tend every day to dignify its noble possessor, prepare and mature him for immortality? What, I repeat it, are illustrious titles and unbounded affluence, without this divine spirit, but the momentary radiance of the same morning dews, which glitter awhile, and then vanish for ever?

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