No wonder, then, that Wells' "Essay on Dew" is famous. But among the thousands of readers of Herschel's discourse, probably not half a dozen have ever seen, much less read, Wells' essay. The Library of the British Museum has a copy of the second edition, and a few of the public institutions of the metropolis have copies in their libraries; but it is seldom or never met with in private scientific libraries. We have often been surprised that the fame of the work should not have led to its republication.

This essay, then, is cited as a model of inductive experimental inquiry; that is, the author is supposed to have taken up a subject, which was but obscurely known, or known erroneously, and guided by one or two leading ideas, to have instituted a number of experiments which, by their teaching, suggested others, and finally landed him on the domain of sound theory, as respects the subject in hand. Now, in a case of this kind, it is not necessary to the author's fame or originality that all his experiments should be new; it is not only right, but desirable, that he should take advantage of the labours of his predecessors and contemporaries, and enlist into his service every stray fact that is likely to assist his inquiry. But it is necessary, in adopting this course, that he carefully adhere to the law of meum and tuum; and that, when he comes to inform the world of his labours, he point out what others had done before he began his experimental inquiry; sum up honestly their results, with ample reference to books and memoirs, and show that such and such was the state of the question as he found it, and such the condition of the inquiry when he ceased to pursue it.

But surely Dr Wells adopted this latter course in his famous essay, if indeed there were anything to point out on the subject of dew except what he himself discovered. The numerous works on natural philosophy do indeed favour the common notion that Dr Wells is the author of the modern theory of dew, and that he arrived at it by the induction of a series of beautiful experiments of his own contrivance. Writers on popular science are accustomed to distil their books from those of their predecessors, and to neglect that important part of the distiller's art, namely, rectification. Now this process of rectification can NEW SERIES.-VOL. XIII. NO. 1.-JAN. 1861.

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only be conducted by referring to original memoirs, and these it is not easy to find, without considerable practical knowledge and acquaintance with the literature of the subject. Those writers who so constantly refer to Dr Wells' essay with applause, are little aware that most of its results had been published long before the author commenced his labours, and that the theory for which he has obtained so much credit, was also similarly indicated, in brief but unmistakable terms.

Now, if this statement be true, the very first place in which we ought to look for a confirmation of its truth is in the essay itself. But there we find only a loose and general reference to authorities, and a very scanty acknowledgment of other men's labours. Indeed, the style of the essay is that of a man who is announcing original discoveries. We will give one specimen, and would ask the reader whether such language as this does not entirely preclude the notion that such observations had ever been made before? He says:-"I have frequently seen during nights that were generally clear, a thermometer lying on the grass plot rise several degrees, upon the zenith being occupied only a few minutes by a cloud. On the other hand, I observed a very great degree of cold to occur on the ground, in addition to that of the atmosphere, during short intervals of clearness of sky, between very cloudy states of it."

Circumstances have led us to inquire into the history of the theory of dew, and the result of our investigation may perhaps employ half an hour of the reader's time not unprofitably. Our purpose must not however be misunderstood. We are not anxious to detract from the real merit of Dr Wells, nor to cast the smallest pebble against his admirers. The Essay on Dew is an elegant production, and proves its author to have been an earnest inquirer into nature-a good observer; and, if not quite so original as is generally supposed, we believe him to have been conscientious, and that while treading in other mens' footsteps he sometimes fancied himself to be cultivating his own clearing.

Now, in order to investigate Wells' claim to be regarded as an original discoverer we will credit him with the following six items, and then proceed to strike a balance by a careful

inquiry into the debits. The chief points, then, which Dr Wells is said to have established, may be thus stated:

1. That on clear and serene nights the surface of the earth is colder than the air some feet above it.

2. That on such nights dew, or hoar-frost, is formed. 3. That in cloudy weather the temperature of the ground approaches, and is often identical with, that of the air; and under such circumstances little or no dew is formed.

4. That screens, even of the lightest material, interposed between the ground and the clear sky, and in general whatever interrupts the view of the sky, prevents that portion of the ground thus protected from cooling below the temperature of the air.

5. That different bodies exposed to the clear sky become colder than the air-the times and amounts of cooling being in general different in different bodies.

6. That all these varied phenomena are to be accounted for on the principles of radiation and condensation, by the first of which the surface of the earth after sunset, provided the sky be clear, cools down below the temperature of the air; and by the second of which the vapour suspended in the air is reduced to the liquid state by contact with a body cooler than itself. But should the sky be clouded, or the ground be protected by means of screens, the heat radiated from the earth is reflected back again, and thus maintains the surface at or about the same temperature as that of the air.

The history of dew is a good illustration of the vicious habit of transposing cause and effect. The ancients having noticed that dew was most abundant on moonlight nights, supposed the moon to be the cause of the dew, and the poet embalmed that belief in the term Rorifera Luna. Aristotle nevertheless more accurately described it as a species of rain, formed in the lower atmosphere in consequence of the moisture which had been evaporated by day being condensed by the cold of night into minute drops. Bacon (Natural History, p. 866) noticed that starlight and bright moonlight nights are colder than cloudy nights. Muschenbröck regarded dew as a real perspiration of plants. Du Fay considered it to be an electrical phenomenon, since metals contract it but feebly, and metals

being good conductors convey away the efficient cause of the phenomenon. The common notion, however, was that dew rose out of the ground. These are mere fancies, not steps in our history, and it is surprising that they should have been conceived, seeing that a certain amount of correct observation had already been recorded. The first point to be decided was, whether dew rose from the ground, or whether the moisture that supplied it already existed in the air. The Florentine Academicians decided that the moisture was in the air, and they even attempted to estimate its amount by cooling the outer surface of a conical glass by filling it with ice, and then noting the moisture which condensed on the surface, and endeavouring to estimate its amount. This was done more accurately by the Hon. Robert Boyle, who, in his "Experimental History of Cold" (published in 1665), determined experimentally that the beautiful exhibition of frost on the window pane is "generated of the aqueous corpuscles that, swimming up and down in the air within the room, are by the various motion that belongs to the fluid bodies as such, brought to pass along the window, and then by the vehement cold of the neighbouring external air, communicated through the glass, condensed into dew and frozen into ice." To prove this, Boyle sealed up in a phial a freezing mixture of snow and salt. "After a while, the salt beginning to melt the snow, the dew on the outside began to congeal, and being rubbed off, the hoar-frost would quickly begin to come again. This phial, for further trial, being put into a pair of scales with a counterpoise, after a while, as the vapours that wandered through the air in the warm room happened to be detained more and more upon the outside of the glass and to be there frozen, the scale wherein the glass was began to be deprest, and to shrink lower and lower; after which, by adding a little to the counterpoise, we reduced them again to an equilibrium, and yet after a while the scale that held the phial subsided again more and more." In this way Boyle collected on one occasion 18 grains of water, and on another 20 grains.

In this experiment Boyle distinctly recognises the fact, that dew and hoar-frost are formed by the precipitation of the vapour of the air upon a colder body. Looking back upon this

experiment, it seems so easy for Boyle to have transferred the scene of his operations to the surface of the earth sparkling with dew, or bedecked with hoar-frost, and to have reasoned thus-" After sunset, the earth must be colder than the superincumbent air, or why this dew and hoar-frost ?" But before the question could assume this simple form, there was still more than a century of observation and experiment, doubt, difficulty, and discussion, but nevertheless of steady progress to be gone through.

A real advance was however made by Le Roi* of Montpellier in 1752, who (apparently without being acquainted with Boyle's experiments) was led to doubt the truth of the received notion that dew rose from the ground, by the well-known experiment of putting ice into a dry glass in summer, when dew forms on its outer surface. Such moisture, he rightly thought, must be deposited by the air; and to test his opinion he sealed a bottle of white glass containing air at the temperature of 20° Réaumur. As evening came on and the temperature of the air declined to 15°, the interior of the bottle was bedewed in the upper part; when exposed to the cold of night (which was as low as 6° R.), there was a considerable deposit of dew within the bottle. On the next day, when the bottle shared in the warmth of the returning sun, the whole of the moisture was taken up again, and the bottle became as transparent as it had been the day before.

Le Roi also reasoned correctly on this experiment, that the quantity of moisture in the air is dependent on the temperature. Hence, by lowering the temperature there must always be a certain degree of cold at which the air will deposit a portion of its moisture, and he named this temperature the point of saturation of the air (degré de saturation de l'air). "To determine this point," he says, "I take water sufficiently cooled to precipitate the moisture of the air on the exterior surface of the vessel that contains it. I pour this water into a large glass, very dry on the outside, and plunge into it the bulb of a thermometer in order to note its temperature. I allow it to become warmer by half a degree, and then pour it

*Sur l'Elevation et la Suspension de l'Eau dans l'Air, et sur la Rosée." Mémoires de l'Academie Royale des Sciences, 1752. Paris, 1755.