and relative situation of such parts of the grained earth-stone, it becomes evident that they are the visible parts of a concentric layer which underlies, .but is united to, the compact layers. Let us instance the primitive district in Cornwall and Devonshire. If a person views the section of the largest granite hill near Redruth, taken in a straight line towards Dartmoor, in this direction the surface of the granite descends regularly till covered with the compact layer; then this layer descends also in the same direction, for a certain distance, but not so steep as the grained layer, or granite. Let this person now remove himself to Dartmoor, and take a sectional view of this hill in a straight line to the granitic hill near Redruth. The Dartmoor granite descends towards Redruth precisely in the same way as the Redruth granite does to Dartmoor, till it is covered by the compact layer; which also declines to the place where it dips to from Redruth. Now the inference is, that these are two of the highest parts of a granitic layer which lies totally under the compact layer in this part of the country; and the fact is, that the granite and granitic veins, which proceed from it, are found in the intermediate space between Redruth and Dartmoor. The same conclusions may be drawn if a person looks towards the granite near the Land's End, or towards the Bristol or British Channels. Hence these granitic masses are but the visible parts of a grained layer that lies under the compact layer in this part of the world. Now as all granite or other grained parts present the same appearance as these, and exist in every elevated part of the earthstone, the grained layer which is found in Cornwall is but a part of such a layer that continues in every direction round the world, and appears in every country, in consequence of having an undulating surface, whose eminences reach above the compact layer. Finally, specimens of the different varieties of grained earth-stone, such as granite and sienite, approach so near one another in appearance, that it requires the most expert mineralogist to determine with precision to which variety some of them belong. There, therefore, appears to be no difficulty in the way in supposing that granite and sienite are portions of the same mass; on the contrary, this deduction might have been drawn from their approximating appearances.

The undulated surface of the earth-stone was produced in the following manner. The process of consolidation proceeded upwards from below till it reached the earth's surface. Before it had acted on the grained layer, a number of inequalities commenced in the surface, which divided the solid from the fluid matter, because the matter below contracted more, and in consequence sunk lower in one place than in another; as the consolidation continued upwards, these inequalities increased, and at the surface gave rise to the undulations which I have described, when speaking of the earth's features, as being peculiar to primitive districts, and also produced such undulations as contain a series of stratified formations in their lower parts. The hollow between the top of Keswick Mountains,

and the top of those in the Isle of Man, is the lower part of one of the last-mentioned undulations. There would be a time when the undulations in the surface between the solid and fluid matter were so great, that the latter matter would retire into hollows, and stand just as high as the highest parts of the former matter. In the continuation of the process, the elevated parts of the solid earth-stone would gradually extend above the surface of the earth-stone's fluid matter; because the hollows in the surface of the solid matter would increase in size, and the still fluid matter retire into them. In this way the fluid portions of the compact layer got below the higher parts of the grained layer; and till the consolidating process reached the general· surface of the compact layer, the fluid matter kept sinking, and its surface retiring downwards, and leaving a part of this layer solid and united to the grained layer; but when nearly the whole of the compact layer had obtained the solid form, its surface assumed throughout a series of small undulations, whose dimensions continued to increase till the matter had reached its present degree of solidity.

2. Of Concrete Lime-stone.

This matter comprehends all those grained and compact limestones and chalk which are not stratified; and includes most of the primitive, transition, and floetz, lime-stone and chalk, formations of Werner.

The matter of the lime-stone while in a fluid state completely covered the earth-stone. When the compact layer had assumed the solid form, the consolidation of the lime-stone layer commenced. But previous to this event the fluid calcareous matter had first sunk gradually below the tops of the grained layer, and then below the higher parts of the compact layer. As the inequalities in the surface of the earth-stone are very great, the universal continuity of the lime-stone was destroyed, and parts of it, while still fluid, were kept back in hollows; and parts arrested by the consolidating process as they were retiring downwards. It is, therefore, found either bending round hollows of the earth-stone, or only lying in parts on one of the sides of such hollows. We are now come nearly to the conclusion. Had there been no inequality in the contraction of the earth's matter after it had assumed the solid state, the grained and compact earth-stone layers, and the lime-stone layer, would have been completely hid by a stratum of water equal in quantity to the present ocean, which last may be considered the remains of that fluid from which the lime-stone proceeded; by the same rule the stratified formations could not have existed. But as the unequal sinking produced hollows, and exposed the earth-stone and lime-stone to view, and as the ocean retired into, and was again forced out of, these hollows to its present situation, by the slow and progressive accumulation of stratified formations, this unequal sinking of course gave rise to a variety of formations. Now

as the unequal sinking is a consequence of the unequal contraction, the phenomenon of formations must be "a consequence of the unequal contraction of the earth's matter."

Having now finished what I proposed, I will close these essays with some general remarks on stratified formations. I consider the great formations, namely, the sand-stones and coal formations, to be formed in the manner that lakes are at present filling up. The sand-stones are large bars deposited by former rivers at the places where they entered former lakes, and the coal formations are the sediments of these lakes.

I scarcely need remark how well the position, relative situation, and the nature of these formations, accord with this idea; nor need I show with what facility this theory at large accounts for the existence of entombed marine animals so high above the present ocean; nor why these bodies are in general confined to lime-stone, the large vegetables to the sand-stone, and the small to the coal formations; as these phenomena admit of the easiest and clearest explanation upon the principles displayed in these essays.

ARTICLE XI.

Extract of a Letter from Mr. Benedict Prevost, Professor at Montauban, to Mr. Pierre Prévost, Professor at Geneva, respecting the Dew which is deposited on that Side of Panes of Glass where the Air is coldest.

THE annexed abstract of a letter from Mr. B. Prevost relates to a fact which has been discussed in your Journal (vol. vi. p. 379, 432).* I think your readers will consider it as necessary to complete the discussion in question.

(Signed)

Geneva, Jan. 1816.

I am, &c.

P. PREVOST.

As to the humidity which is deposited on the outside of a glass pane, though the air on the outside be colder than that within, I am certain that I have observed it several times, pretty frequently indeed. I mean that I have observed this humidity on the outside when the external thermometer was lower than the internal. you know, my dear cousin, and you state it yourself (at No. 24 of your § 192, p. 241), that I endeavoured to procure thermometers with flattened bulbs, and sufficiently sensible to point out the diffe

* See also the present Number, p.

But

rence of temperature of the sides of the glass at the same time, which I think would have given me instructive results; but I was not fortunate enough to succeed.

I mark down with regularity, in a journal containing the height of the internal and external thermometer, the dryness or moisture of the glass. I find in this journal several confirmations of what I have said. I even find some days in which the glass was wet without and moist within.

It may be objected to me that these observations prove only the co-existence of this external humidity, and an external temperature lower than the internal; and not that this external humidity was deposited while the external temperature was lower. I must acknowledge that I do not recollect to have seen the moisture depositing itself on the glass, while the external temperature was lower than the internal; and I cannot at this moment, for reasons that I shall mention to you afterwards, consult the original journal of my experiments. But the following observation, which I have an opportunity of repeating every year, does not appear to me to agree with the empyrical principle established by Dr. Wells, "that bodies are not covered with dew unless they be colder than the air." The window shutters of the country house in which my experiments on dew are made are painted green with oil paint. When open, they are applied (at least in part) to the wall of the house. Now in the cold season, when the nights are fine, they are often so covered with moisture as to be dropping with water in the morning, while the air has been getting colder during the whole time, as is shown by thermometers fixed against them. This happens principally when the preceding day has been fine, especially if it has been hot. But as in this case the external air deposits moisture on these bodies while cooling, it follows that dew (or humidity) is deposited on bodies hotter than the air which surrounds them, and which gives out this moisture.

It will be said, perhaps, that it is the dry and cold air of the higher parts of the atmosphere which, descending during the night, produces the cold, while the humidity is deposited from the hot and moist air which ascends. But upon the whole the air which descends uniting with the ascending air must always cool it. That air accordingly is colder, &c.

My chamber looks towards the north, and the roof of the house projects some feet over the building, so that in winter the ground at the bottom of the wall is in the shade. Though I have a fire in the room during the day, and often till pretty late at night, if in the evening I shut the shutters, though I leave the window open, still a great deal of moisture is deposited on the outside in circumstances similar in other respects to those mentioned above. Here the external air appears to be colder than the window shutter, and yet it deposits a good deal of humidity on it. Yet, as I have already observed, all the ironwork of the shutter, whether painted or not, open or shut (with the exception of some pendent or very

salient parts, not painted, and covered with black oxide, as iron usually is) remains absolutely dry. Among other parts, the heads of nails, whether higher than the surface of the wood, or sunk below it.

I may observe, likewise, that the water deposited on the shutters during the night is often exhaled in the morning in the midst of a thick fog.

Perhaps in explaining these phenomena we ought to consider separately the effect of the cold air which descends, and of the hot vapour which ascends. The first probably cools the surface of the shutter only to a very minute depth, in consequence of its inconductibility, so that its temperature is lower than that of the vapour which ascends, although the air of the room and the inside of the shutters be hotter than this vapour. This seems to be confirmed by the dryness of the ironwork. This, I conceive, would make my observations agree with those of Dr. Wells. In that case, both of us ought to modify a little our general formula. The air has often less to do with these phenomena than would appear at first sight. As there is probably no action between the molecules of air and those of vapour (except what those of air may produce on each other, and those of vapour on each other; and philosophers, I believe, are sufficiently agreed on the subject), while no chemical combination takes place, it is possible that the particles of aqueous vapour distributed through the cold air may preserve a higher temperature for a time long enough to rise to a certain height; so that the air in the neighbourhood of the window shutter, though colder than it, may notwithstanding contain a hotter vapour, &c.

I here subjoin some observations found in my meteorological journal after the above was written, by inspecting it more carefully.

The inside thermometer is suspended against the glass itself.

4.8

4.6

4

4.3