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nature far greater even than that which it is brought to explain." The main difficulty which seems to strike all those who have hitherto considered the subject, appears to lie in the method of getting rid of the waters of the Deluge. Many will grant you that they came, if you can show how they departed. Amidst all the conjectures that have been offered on this point, sufficient stress does not appear to have been laid upon the idea, that they may not have departed; but that the waters which "were increased greatly upon the earth" are still with us. I shall offer a few remarks upon this subject, rather with a view to promote future inquiry, than with the wish to propose a new hypothesis. I shall assume that the flood which we are informed prevailed for 150 days, consisted of waters at that time added to the earth, and leave to the future consideration of geologists, whether the supposition of their having been partly absorbed by the solid portion of the earth is not of itself a cause sufficient to explain the present state of the surface of our planet.

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Suppose the original level of the surface of the ocean to have been the line A, and an increase of waters to have raised the surface from A to B, sufficient to cover the tops of the highest mountains; I would ask whether, if the increase were rather sudden as it is stated to have been, we may not imagine that a considerable depression below the highest level would afterwards take place, owing to those solid portions of the earth which were not originally covered, becoming saturated with moisture; and thus, after a certain lapse of time, the surface of the ocean might rest at C, leaving the higher summits of the old continents again exposed. To decide whether or not such may have been the case will of course require that future observations should be made with this object in view. There are, however, certain facts already noticed in geology which tend to show that an increase of elevation above the original surface of the ocean has actually taken place; such as peat land, containing vast numbers of trees, which are found in some situations extending under the bed of the ocean, and whose destruction appears to have been coeval with the Deluge. Also the swampy condition of large tracts of fen country, which are now incapable of producing timber, but in which immense quantities of the largest growth are found buried in a sound state. If it should be objected that the mass of waters brought upon the earth was far too great to admit of the supposition of their having been absorbed to an

extent sufficient to allow for the present altitude of the highest mountains above the surface of the ocean, I reply that this must be a subject for future observation; but I think this difficulty, which at first sight appears so startling, will diminish considerably by considering what would be the quantity of water sufficient to cover the tops of the highest mountains at this present moment. The greatest altitude of any known mountain is about five miles, which is but trifling compared with the radius of the earth, which is above 4000 miles. Let a person take a three-inch globe in his hand, and consider how thin would be the film of water sufficient to cover the particles of fine dust which are attached to its surface. It should seem also that those portions of the earth which were partly saturated by moisture before the Deluge would absorb a still greater quantity upon the rise of the ocean, and thus a further diminution might be accounted for. Suppose a basin made of plaster of paris, chalk, or any other porous material, to be partly filled with water, the sides will immediately imbibe a certain portion, and the surface will fall. When the water has reached its greatest depression, fill the basin, and you will find the sides still capable of absorbing an additional quantity, and the surface will not long remain on a level with the rim. I need scarcely observe that the earth appears to be in every part more or less saturated with water. The most solid rocks contain it in great abundance, and the operations of the miner are too frequently impeded by its presence; but I believe that few, if any, observations have hitherto been made with the view of obtaining an estimate of the mean quantity at different depths, or in different descriptions of rock.

This view of the subject perfectly coincides with the account given in Genesis of the gradual manner in which the waters subsided. "And the waters returned from off the earth continually." "And the waters decreased continually until the tenth month." This would hardly have been the case if we are to suppose with some, that there were large empty receptacles prepared for them towards the centre of the earth, into which they suddenly retired.

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I should scarcely venture to allude to the manner in which we may suppose that the waters were brought upon earth, but that I wish to observe upon some other phenomena connected with the supposition of their having been added to the earth's surface, or, in the language of scripture, "increased upon the face of the earth."

Observation appears to have established that the rise of the diluvian waters was gradual, and that with respect to the present surface of the land they came in descending torrents. And this agrees distinctly with one part of revelation, which states, that "the windows of heaven were opened, and the rain was upon the earth forty days and forty nights,"" and the waters prevailed,

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and were increased greatly upon the earth." All which seems to imply an extraneous supply of water; for although the atmosphere at all times contains a certain quantity of aqueous vapour, yet this would not be sufficient to answer the demand. know that the weight of the whole atmosphere is equivalent only to a depth of water of about 34 feet, and this is made up of atmospheric air, water, and different gaseous mixtures. We must, therefore, look for some other cause, which has ceased to operate since the supply was furnished; and here of course nothing but conjecture can be offered. With many of my predecessors in this department, I must have recourse to one of those bodies which have so often been considered as the probable cause of the Deluge, though the mode in which they have been supposed to operate in effecting that event has as often been refuted or ridiculed. But before we attempt to enlist so myste-" rious an agent into our service, let us inquire what it is we actually know respecting the nature of comets.

Of this we can judge only from the astronomical and optical phenomena which they present. It is certain that they are material substances, and it appears universally conjectured by the most accurate observers, that they are in great part, if not wholly, composed of aqueous vapour. Some comets present a nucleus encircled by this vapour; others have no nucleus at all. As they approach the sun, they become brighter; the luminous train or tail, when it exists, becomes enlarged and more brilliant, and when the comets have arrived at their perihelion, their lustre is sometimes found to exceed that of the planets. In their retreat from the sun, these phenomena are reversed, till at length the light reflected from them is too trifling to be any longer visible. Although the opinions which have been promulgated concern ing the tails of comets differ materially with respect to what may be the nature of their substance, yet they are all compatible with the idea of their nuclei being composed of aqueous parti cles. One opinion is that these tails are the light of the sun refracted through the comet acting like a transparent lens ; but this idea seems to have been satisfactorily refuted by subsequent observation. Others suppose them to be the vapour of the comet, either driven behind it by the impulse of the sun's rays, or raised by the heat of the sun; the latter opinion, which was held by Sir Isaac Newton, seems also compatible with the idea of the comet transmitting the rays of light, since the heat would be greatest, and consequently the vapours lightest, along the train of light reaching from the nucleus to the focus of this astronomical lens. However this may be, let it be granted as highly probable, that some comets are composed of aqueous particles, which at a distance from the sun will probably concrete into the form of a globe of ice, and on approaching him will either be wholly or in part converted into vapour. What will be the effect of such a comet approaching within the sphere of the earth's attraction?

Before replying to this question, I will ask what has been the fate of the comet of 1770, whose periodic time was not greater than five years and a half, and which could never wander so far from the sun as to get beyond the orbit of Saturn? Yet this has not been since that time, and no solution of the cause of its disappearance appears so probable as that offered by Dr. Brewster," that it now exists under the form of those enormous atmospheres which accompany Ceres and Pallas."

If it be too much to suppose that the nucleus of such a comet, though composed of aqueous particles, would fall to the earth, we may, perhaps, conjecture that a portion of its nebulous train becoming entangled in our atmosphere would be attracted to the earth, and descend in the form of rain upon every portion of its surface successively, as the earth turned upon its axis. This conjecture as to the mode of supply is here mentioned, as I before stated, with the view of promoting another inquiry upon a point which seems to be pretty well established in geology, viz. that the mean temperature of the earth's surface, at least in these latitudes, has been very sensibly diminished ever since the Deluge. If the mean temperature of the earth's surface depend upon the distance of the centre of the earth from the surface of the ocean, then the increase of waters brought by the deluge would, by increasing the radius of the earth, produce the phenomenon which has been observed.

It may also be proposed as a subject of inquiry, what would be the effect produced upon the atmosphere by increasing the proportion of aqueous surface to that of the dry land. Would the atmosphere become more highly charged with aqueous vapour, and cause a greater quantity of rain to fall annually? We know that some of the planets possess an atmosphere of extreme purity, and that others apparently have none. Among the former number is the moon. Now it is remarkable that the latest discoveries lead us to suppose that the moon possesses no seas, though there are large indentations on her surface which would speedily become such, were she inundated by a Deluge.

How far such investigations as these may tend to confirm the history of the rainbow having been first seen after the Deluge, or, in other words, the non-existence of rain previous to that event, I leave to the inquiry of meteorologists, requesting them to bear in mind that the only account we have of the method by which the earth was refreshed before the Deluge is, that "there went up a mist from the earth and watered the whole face of the ground;" "For the Lord God had not caused it to rain upon the earth."

I remain, Sir, your obedient servant,

J. S. HENSLOW.

ARTICLE V.

Facts, Observations, and Conjectures, relative to the Generation of the Opossum of North America. In a Letter from Prof. Barton to Mons. Roume, of Paris.

(To the Editor of the Annals of Philosophy.)

DEAR SIR,

Penlerrgare, Sept. 3, 1823.

I RECEIVED from its author, the late Prof. Barton of Philadelphia, the enclosed printed copy of a letter which I have never elsewhere met with, and it relates some circumstances which have not been noticed by Sir Everard Home in his valuable observations on the generation of the marsupialia in the Philosophical Transactions for 1808, 1810, and 1819.

The fossil remains of a species of didelphis are said to be not unfrequently found in the Stonesfield slate, and I know of no other animal belonging to either of the secondary or any older formation which possesses the smallest claim to be called viviparous, nor does even this family in its mode of generation, appear to be more than one of those links which connect the higher order of viviparous with oviparous animals.

In this point of view the Professor's letter becomes interesting not only to the zoologist, but in some degree to the geologist also; and I, therefore, offer it to you for insertion in the Annals of Philosophy. I am, dear Sir, yours truly,

DEAR SIR,

L. W. DILLWYN.
Philadelphia.

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In looking over my list of correspondents, I find that I am indebted to you a letter. I cannot think of writing a mere formal letter of apology, for my long silence; and, therefore, shall contrive to send you something that may, at least, amuse you.

You and I have often talked together, and speculated, about the generation of the Opossum of North America (the Virginian Opossum of Pennant; my Didelphis Woapink).* I think I

* There is not a little confusion concerning the nomenclature of the different species of Didelphis, in the writings of Linnæus, Gmelin, and other naturalists. See the articles "Didelphis marsupialis," and "D. Opossum," in the Systema Naturæ, as published by Linnæus himself, and by Gmelin. I have, therefore, thought it most advisable to impose a new and more determinate name upon the animal, which has been the subject of my experiments. The specific name of marsupialis is not very happily applied to any particular species of Didelphis, since most of the species of this singular genus are furnished with the marsupium, or abdominal sack. I object to Dr. Shaw's specific name, Virginica (taken from Mr. Pennant), because it implies, that our Opossum is restricted to, or especially common in, Virginia; whereas this animal is nearly equally common in every part of the United States (east of the Missisippi), from the latitude of 40 to that of 25, and even much further south. The name Woapink, which

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