experiment, establishes the proposition that bodies become colder than the neighbouring air before they are dewed; and as different bodies project heat with very different degrees of force-"in the operation of this principle, conjoined with the power of a concave mirror of clouds, or any other awning, to reflect, or throw down again those calorific emanations which would be dissipated in a clear sky, we shall find a solution of the most mysterious phenomenon of dew."

They who do not possess Dr. WELLS's Essay, and who are desirous to investigate and scrutinize the means which have been made use of to ascertain the causes of the phenomenon of dew, will probably be gratified by an abridged account extracted from the "Encylopædia of Gardening," of those experiments and observations which have led to the establishment of the present theory. I shall mark those passages in italics, which contain important facts that promise to throw some light upon the general subject under inquiry, and to lead to deductions very different from those which have been drawn from them.

"After a long period of drought, when the air was very still, and the sky serene, Dr. Wells exposed to the sky, twenty-eight minutes before sunset, previously weighed parcels of wool and swansdown, upon a smooth, unpainted, and perfectly dry fir table, five feet long, three broad, and nearly three in height, which had been placed an hour before in the sunshine, in a large level grass field. The wool, twelve minutes after sunset, was found to be 14° colder than the air, and to have acquired no weight. The swansdown, the quantity of which was much greater than that of the wool, was at the same time 13° colder than the air, and was also without any additional weight. In twenty minutes more, the swansdown was 14° colder than the neighbouring air, and was still without any increase of its weight. At the same time the grass was 15° colder than the air four feet above the ground."-(Encyc. of Gard., No. 1245.)

The facts in this extract which require attention are:-first, the state of the air: it was still, and the sky serene a state which corresponds sufficiently with that described at No. 138 (on the Atmosphere). Second,—the nature of the materials employed: they were wool and swansdown,-both of them electrics, and ranking among the worst conductors of heat and electricity; they also were placed upon a fir table, that had been rendered an INSULATING medium and a non-conductor, by the previous drying, as well as by the resinous, electric matter which fir contains. These electrics, these non-conductors, then, are thus insulated at an elevation of three feet above the surface of grass, which becomes 15° colder than

a stratum of air at a distance of four feet above the ground, and one foot above the table. Under these circumstances, the swansdown becomes cooled to within of the grass (which most likely was dewed, though it is not so stated), and yet remains dry.

The deduction to be drawn from these facts is, that some nonconductors do not receive the dew, although cooled 144° below the temperature of the neighbouring air; consequently, that the precipitation of dew cannot depend solely upon the reduction of temperature, nor upon atmospheric moisture independent of the agency of vegetable organization.

196. A very slight covering will exclude much cold." Being desirous," says Dr. Wells, "of acquiring some precise information on this subject, I fixed perpendicularly in the earth of a grass-plot, four small sticks, and over their upper extremities, which were six inches above the grass, and formed the corners of a square, the sides of which were two feet long, drew tightly a very thin cambric handkerchief. The temperature of the grass, which was thus sheltered from the sky, was always found higher than that of the neighbouring grass, which was uncovered, if this was colder than the air. One night, when the fully-exposed grass was 11° colder than the air, the latter was 3° warmer than the sheltered grass; and the same difference existed on another night, when the air was 14° warmer than the exposed grass. One reason for the difference, no doubt, was, that the air, which passed from the exposed grass, by which it had been very much cooled, to that under the handkerchief, had deprived the latter of part of its heat; another, that the handkerchief, from being made much colder than the atmosphere, by the action of radiation of its upper surface to the heavens, would remit somewhat less heat to the grass beneath, than what it received from that substance." "A difference in temperature of some magnitude was always observed on still and serene nights, between the bodies sheltered from the sky by substances touching them, and similar bodies which were sheltered by a substance a little above them."

Dr. Wells found that grass sheltered by a cambric handkerchief, raised a few inches in the air, was, on one occasion, 3°, and on another 4° warmer than a neighbouring piece of grass, with which the kandkerchief was in contact.

These extracts contain facts of real practical utility; but the philosophy of the reasoning, and of the inferences drawn, is doubtful; the whole of the phenomena are ascribed to the influence of radiation acting upwards, and again in a contrary direction, as if by volition. This taking of effects for causes, will not, however, prove

of any detriment to the practical utility of the observations, as far as they may be brought to bear on the protection of fruit trees; and it will not be amiss to compare them with the paragraph on the "protecting of wall fruit," at No. 86. Article, Peach.

197. Heat from a Covering of Snow.-Dr. Wells ascribes the benefit derived by vegetation from such a covering, in extremely cold latitudes, to the prevention of radiation, and not to the protection from the access of a more intense degree of cold existing in the atmospheric regions, "while low vegetable productions are prevented, by their covering of snow, from becoming colder than the atmosphere, in consequence of their own radiation, the parts of trees, and tall shrubs, which rise above the snow, are little affected by cold from this cause; for their outermost twigs, now that they are destitute of leaves, are much smaller than the thermometers suspended by me in the air, which in this situation seldom became more than 2° colder than the atmosphere."

The larger branches, too, which, if fully exposed to the sky, would become colder than the extreme parts, are, in a great degree, sheltered by them; and, in the last place, the trunks are sheltered, both by the smaller and larger parts, not to mention that the trunks must derive heat by conduction through the roots from the earth, kept warm by the snow.-(From Essay on Dew,-See Encyc. of Gard., No. 1204, 5, and 7.)

That radiation may take place from a heated surface into a cooler medium, no one can well doubt; a basin, or wide-mouth glass vessel inverted, will receive moisture from the surface of the ground, and exhibit it in a condensed state; but it will equally receive it from dry, sandy, unclothed ground, as from a surface covered with herbage; radiation so considered, would be little else than another term for evaporation-a process which implies the decomposition of water, and its conversion into vapour. The radiation alluded to is, it should appear, supposed to be produced by the agency of vegetable organized beings, and to result from a faculty which they possess of conducting or carrying off heat from the surface of the earth, and propelling it from their extremities, into the aërial medium with which they are surrounded. But have not the advocates of this theory of radiation overlooked, or lost sight of, the peculiar structure of vegetable bodies, and of all those electromagnetic agencies which are in constant operation throughout the elements of nature?

In allusion to Dr. Wells' remark upon trees, their vital power, and with it, that degree of heat which is essential to their preservation, remain unascertained: whether evergreens are worse conductors

than deciduos suhrubs and trees, may not be determinable, but certain it is, that by the severe frost of January, 1838, when, on the 20th, the mercury fell to 2°, and in places to 6o below zero, the Laurustinus, Bay, Arbutus, and some laurels, received much more serious injury than any deciduous tree or shrub that came under my observation.

If the reader review what has been said at No. 54, and compare it with the foregoing quotations, he must perceive that vegetables are, indeed, most important instruments of conduction, and that they are constantly employed in regulating the state of atmospheric electricity. If he investigate the structure and uses of all those pointed terminations of the leaves and their serratures: of the divisions of the corollas and calyces of flowers; of the thorns, prickles, and bristles of every description; he may find ample cause for believing that they are all destined to perform very important offices connected with vegetable vitality; and thus, he, too, may be "led from a copious induction of facts, derived from observation and experiment," to conclude, that vegetable bodies of all descriptions, grass, herbage, shrubs, and trees, those "best of radiators," which are "the soonest dewed," constitute, in fact, an assemblage of so many conducting points, that they are not only the best conductors of electricity, but dependant upon its agency, under one modification or another, for the propulsion, laboration, and distribution of their own vital and secreted fluids. (See Nos. 61, 62, and 63.)

198. It will now be interesting to compare what has been quoted, with a short extract from the Natural History of Selborne, by that observant naturalist, Mr. White, on the attraction of trees.

"In heavy fogs, on elevated situations especially, trees are perfect alembics; and no one that has not attended to such matters can imagine how much water one tree will distil in a night's time, by condensing the vapour which trickles down the twigs and boughs, so as to make the ground below quite in a float. In Newton-lane, in October, 1775, on a misty day, a particular oak, in leaf, dropped so fast, that the cart-way stood in puddles, and the ruts ran with water, though the ground in general was dusty."

"Trees perspire profusely, condense largely, and check evaporation so much, that woods are always moist; no wonder, therefore, that they contribute much to pools and streams. That trees are great promoters of lakes and rivers appears from a well-known fact in North America; for since the woods and forests have been grubbed and cleared, all bodies of water are much diminished: so that some streams, that were very considerable a century ago, will not now drive a common mill.-(Vide KALM's Travels in North America.) Besides, most woodlands, forests, and chases, with us, abound with

pools and morasses; no doubt for the reason given above.”—(Vol. I. 346 and 348.)

If these be credible facts, can we reconcile them with the hypothesis of radiation of heat. Can the phenomena be explained by that hypothesis? Why should trees and herbage condense such a volume of water? Why should a spot of freshly digged ground be covered over with hoar frost, when hard, unwrought ground, close to it, shall not exhibit one particle of rime, unless it be on spots where some weeds or projecting points may happen to be standing above the surface? These are, to the reflecting mind, subjects for deep interest and inquiry! However, if vegetable bodies become dewed, because they are the best radiators, and if they themselves be vehicles of electric currents; if, moreover, the flow and direction of their vital and secreted fluids be governed by the influence of those currents, then, connecting these facts, we must infer that the radiating power of plants, and their covering of dew depend upon the instrumentality of that agent which induces the flow of the ascending current of the sap, and the expulsion of the perspirable matter from the vessels of their leaves; and if so, the deposition of dew, whether derived from atmospheric moisture, or from that exuded by the plant, must of necessity be considered an electrical phenomenon.

199. The deduction from all that has been stated is this,—we are to look for the solution of this wonderful phenomenon of dew to the agency of that principle, which induces not only the radiation of heat, the decomposition and evaporation of water, and its conversion into atmospheric air, but also those changes which are in perpetual operation in the aërial ocean. In the sections on WATER, and on the ATMOSPHERE, I have entered somewhat at large upon the consideration of those surprising mutations, and therefore need not recapitulate in this place. The proximate or immediate causes of the precipitation of dew upon grass, and herbage, will, I think, be discovered in the peculiar structure and peculiar condition of vegetable bodies. This structure constitutes them, individually or collectively, not only perfect instruments of electric conduction, but also an assemblage of myriads of points, at which the ascending and descending electrical currents meet and neutralize each other, depositing the aqueous particles which, till then, they had held in a state of repulsion, or of infinitety minute division. It does not appear that grass and herbage are endued with the power of radiating or conducting heat, in a degree by any means equal to that of metals-substances which, it is said, do not become dewed at a time, and under circumstances, wherein the circumjacent herbage is covered with minute drops of water a fact, which is not only very remarkable in itself, but one