between the mountain and the river Tandoi was covered to such a depth with bluish mud that people were buried in their houses, and not a trace of the numerous villages and plantations throughout that extent was visible. It

was remarked that the boiling mud and cinders were projected with such violence from the mountain, that, while many remote villages were utterly destroyed and buried, others much nearer the volcano were scarcely injured. At the end of four days a second eruption occurred, more violent than the first, in which hot water and mud were again vomited, and great blocks of basalt were thrown to the distance of seven miles from the volcano."* †

* Lyell's "Principles," p. 430.

Should the reader wish-further than the bibliographical references already contained in this paper enable him—to compare the accounts of the eruptions of other Icelandic volcanoes with those of Kötlugjá, he may consult the following works :—

1. “Annales Islandorum Reg.,” in Langenbeck's "Scriptores rerum Danic. medii ævi," which contains an account of the earlier eruptions in Iceland.

2. "Letters on Iceland," &c., by Uno Von Troil, D.D., Chaplain to his Swedish Majesty, &c. London, 1780; the prefix whereof (pp. 18 and seq.) contains a bibliographical list of 120 works on Iceland, some of which treat wholly or partly of volcanic phenomena.

3. Bishop Finnsen's Efterretning om Tildragelserne ved Bierget Hekla ;" Copenhagen, 1767.

4. "Kort Beskrivelse over den nye Vulcans Ildsprudning i Vester Skaptafjeld's Syssel paa Island i aaret 1783;" being an account of the eruption of the Skaptar-jökul, by Magnus Stephenson, Etatsroed of Iceland, with engravings, 8vo; Copenhagen, 1785. Translated in Hooker's "Journal of a Tour in Iceland," vol. ii. p. 124.

5. S. M. Holme "Om Jordbranden paa Island i aaret 1783;" Copenhagen, 1784.

6. Garlieb, "Island rucksichtlich seiner Vulkane;" Freiburg, 8vo, 1819. 7. Hekla og den sidst Udbrand den Septem. 1845. En Monographie : med 10 Plader Lin. 8vo, Copenhagen, 1847, by J. C. Schythe. A brochure on the most recent Eruption of Hekla, costing about 3s. in this country.

On the Claim of Dr Wells to be regarded as the Author of the "Theory of Dew." By CHARLES TOMLINSON, Lecturer on Science, King's College School, London.

Probably there never was a scientific treatise at once so famous and so little known, as the "Essay on Dew," by William Charles Wells, M.D., published in 1814. A second edition of this treatise appeared in 1815, and a third in 1818, containing the author's autobiography written shortly before his death, which took place in September 1817. The essay excited some discussion during the author's lifetime. Some of the leading facts, together with an epitome of his "Theory of Dew," were at once adopted in books on natural philosophy, and these have been repeated, with little or no variation, by every writer on physics down to the present time.

What then has made Wells' “Essay on Dew" so famous, if at the same time it is so little known? It is but little known, because it has long been out of print, and therefore inaccessible to the readers of popular science; it is famous, because the innumerable books on natural philosophy have referred to it with applause, but chiefly because one of our most celebrated scientific authorities has pointed out this essay as a model of inductive experimental inquiry. Sir John Herschel, in his Preliminary Discourse on the Study of Natural Philosophy," characterises Wells' essay as "one of the most beautiful specimens we can call to mind of inductive experimental inquiry lying within a moderate compass;" and he earnestly recommends it to the student of natural philosophy" as a model with which he will do well to become familiar."*

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* Dr Lardner also, in his "Treatise on Heat" (1833), gives unbounded credit to Dr Wells. He says:—" The result of his inquiries was the discovery of the cause of the phenomena of dew, and affords one of the most beautiful instances of inductive reasoning which any part of the history of physical discovery has presented" (p. 328). In another Treatise on Heat, published in 1855, Dr Lardner still gives the whole credit of the "celebrated Theory of Dew to Dr Wells. Dr Golding Bird, in his "Elements of Natural Philosophy," 4th edit., 1854, also gives the whole credit to Wells. French and German treatises do the same. Thus Eisenlohr "Lehrbuch der Physik," 1860, in describing the capital experiment of two thermometers, one on the ground, and the other in the air, marking different temperatures, says, "Dieser Versuch Wells her," p. 367.

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

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