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blue clay, which produces oysters of a different kind from those found in gravel, being remarkable for the convexity of their shells. Along with these oysters are found belemnitæ, ammonitæ, very small, and saturated with pyritical matter, which gives them a kind of shining-like armature.

On the south side of St. Clement's, the gravel appears again, and abounds with much the same fossils as those on the north side of Oxford. Hard by, in Cowley-Common, are found gryphitæ, or the crooked-bill oyster, of a very large size, and very thick, broader in the margin than those usually called by that name. They are remarkable for shewing the several lamina or stages of their growth, being at first no bigger than a vetch, and proceeding to the size of six inches diameter. Either the world was less populous, or the use of oysters less known in the antediluvian times, than now; for we never find any recent shells arrived to that growth.*

Bullington Green, Headington Heath, Shotover quarries, and the stone-pits at Garsington, all adjacent, are equally replete with great variety of very curious fossils, such as pectines, great and small, echini, belemnitæ, pholades, coralloides, shrimps, claws and other parts of crabs; pinna marinæ, oysters remarkably large and flat, (found recently in Virginia;) naufilitæ, cochlitæ in abundance; a remarkable small serated tree-oyster, auriculares, vertebræ, jaws and teeth of animals, ammonitæ of various kinds, some turbins, strombi, and great plenty of mycetites, astroites, &c.

To close the whole, in our Museum we have the collections of Plot and Lhuyd, which contain great variety both of native and extraneous fossils, which now appear to great advantage, being lately reduced under their proper classes by their present keeper, a gentleman in every respect qualified for the work.

I am, yours, &c.

1757, March.

A. B.

Some Rock-oysters are perhaps an exception to this observation.

XXVI. On the Coluber of Virgil.

Qualis ubi in lucem coluber, mala gramina pastus.
VIRG. Eu. ii. 471.

MR. URBAN,

BY Coluber is here meant not the common snake, but the viper, as is evident from the poet's supposing him to be replete with poison, acquired by feeding upon noxious herbs, whereas the snake is entirely destitute of poison. The venom, in his opinion, was gotten by the serpent's living upon deleterious plants, which is a great mistake, for the viper is carnivorous. However, in the Georgics, lib. iii. 425. et seq. he shews, that he was well aware that the chersydros of Calabria, a poisonous species of serpents, lived upon animal food, such as fish and frogs.

It has been thought, till of late, that the viper had a fascinating power, whereby it charmed its prey into its mouth, being neither quick in its motion, nor having any feet to assist it in the management of any animal that could struggle with it for its life. And it is certain, that this opinion receives great countenance from two papers in the Philosophical Transactions, Vol. xxxi. one by Paul Dudley, and the other by Sir Conrad Sprengell. But the truth is now found to be, as appears by a later narrative from North America, and inserted likewise in the Transactions, that the rattle-snake, which is a species of the viper, gets his prey in this manner. He first bites the animal, and at the same instant the poison pressed out of a bag at the roots of his fangs, runs, through an aperture in the fangs, into the wound; after this he keeps his eye upon the creature, and waits for the operation of the instilled poison; and when it has brought on the death of the animal, he then begins to lick it, and prepare it for deglutition. This is the provision which the all-wise providence has contrived for the subsistence of a serpent, destined to live upon animal food, but incapable otherwise of contending with a creature of any vivacity or strength. But then I would ask, what is it that the common snake lives upon, and how does he get his living? He has many of the properties of the adder or viper, but wants his poison; for I presume it is generally agreed, that the snake is harmless. He is slow, he coils himself, he casts his skin, he sleeps in winter, and is as unable to cope with a

living animal as the viper is. It is said, indeed, that frogs and other creatures have been found in his belly; but the truth of this is what I am desirous of knowing, and as this is the season for their making their appearance after the sleep of the winter, I shall be obliged to any curious naturalist, that will open a few of them this spring, and look into their stomachs, to inform us what he finds there, for at present I can hardly think, if he feeds upon animal food at all, that it can be any thing more than worms and insects; for since he is not armed with poison, it is very difficult to conceive how he can master and manage any larger animal, though his gullet, I suppose, is as capable of distention for the swallowing either of a mouse or small bird, as is the viper's.

1757, March.

Yours, &c.

PAUL GEMSege.

XXVII. On the Phenomenon of Dew.

THE dispute concerning the origin of dew seems as yet to be left undetermined. Some philosophers have insisted that it falls from the middle region of the air, others as strenuously assert that it rises from the bowels of the earth in vapour, which never reaches the middle region of the air, but falls back condensed into water, after having risen a comparatively small distance above the earth's surface.

The former of these allege, in favour of their opinion, "that it is most natural; that we see the rain, which is of the same nature with dew, descending from the superior regions; and consequently ought not to suppose that the dew has any other origin, since it differs no otherwise from small rain, or misling, than in degree. That the atmosphere is continually replete with a vast quantity of vapours; and that, when the solar heat is withdrawn, the cold which occupies the superior regions immediately condenses and precipitates them, if not dissipated by the wind, in form of dew; and that those bubbles or vesicules, though imperceptible to us while separate, easily gather into larger drops (when they fall) by their own attraction; and are, in that state, found on grass, and on the herbs of the field and garden, in the morning, where they remain till they are again exhaled by the sun."

Those of the contrary party say, "That exhalations are continually flying off from the earth; being raised either by

the solar or subterraneous heat, or both. That these eva, porations do not cease even in the night. That, during the heat of the day, these vapours, being specifically lighter than the circumambient air, are dissipated in their ascent; but, in the night, they rise not far above the ground, being immediately condensed and precipitated again by the coid. That though they cannot boast of the universality of their opinion, yet they hope it is established upon a surer foun dation than the other; as they have had recourse to experi ments, the most rigid tests of truth. That M. Dufay, in particular, being resolved to try the grand question whether dew did or did not first ascend in vapour, reduced it to this simple process. He considered, that if the dew did ascend it must wet a body placed lower, sooner than one placed higher, and its under part sooner than its upper; and, upon these principles, he tried the following experiments. He placed two ladders, with their tops resting against each other, their feet at a considerable distance, and their height 32 feet. To the steps of these ladders he fastened squares of glass, in such a manner as not to hang over each other. On trial, he found it exactly as he expected; the lower surface of the lowest square being first wetted, then its upper surface; then the lower surface of the second square; and so on gradually through the whole series."

These are some of the strongest arguments produced on each side in confirmation of each hypothesis. But perhaps neither side has been so fortunate as, upon the whole, to hit upon the true account, nor examined it so narrowly as to preclude any future discoveries. I am, however, apt to believe, after repeated trials, that part of the dew does really fall. I say part, for I hope to make it appear that a great deal of it, perhaps one half, except in thick foggy nights, rises. But when I say, rises, let it be noted that I do not mean in form of vapour; but in manner of perspiration from grass plants, and other herbage; the truth of which position the following experiments will, I hope, in a great measure, put beyond dispute.

EXPER. I. About an hour before sun-set, I inverted a large tub or vat upon some fine fresh grass, and stopped it so close at the bottom that it could have no communication with the external air. Upon examination in the morning I found the grass under the tub, to my surprise, charged as plentifully with dew, as that which was uncovered all around it: but the spherules or drops, though equal in size, were only on the summits of the blades.-N. B. In a windy night there is seldom any dew, or very little; but the wind never affects the

covered grass at all; the drops being as large then, as at any

other time.

II. The former experiment I repeated, but with this addition; under the tub, I suspended a large pane of glass horizontally about a foot, and a little tuft of wool at the same distance, from the ground; I also suspended another pane of glass and another little tuft of wool over the tub, exposed to the air. In the morning I found the grass as before. The glass and wool under the vessel perfectly dry; but that over it

very wet.

III. Made a great many trials on some gross garden plants, such as cabbages, coleworts, brocoli, and several others of the same species, by covering them with the same vessel. In the morning the edges of their leaves were always charged with large round drops; each drop dependent from the extre mity of one of its ribs or fibres. When I traced my finger over the surface of the leaf, I could not be certain whether it was wet or not; but the surfaces of those that were uncovered were bedewed very plentifully.

IV. About ten o'clock in the forenoon, when the dew was all exhaled and the grass quite dry, I inverted the tub again; taking care always, if it was not in a shady place, to cover it with something that might hinder the sun-beams from penetrating; and, in a few hours time, I found the summits of every blade of grass, except those that were withered, laden with as large drops, as they would have been in the same space of time in the night, or perhaps larger. This experiment always succeeded in perfect regularity.

V. At mid-day I made the same experiment on some of the before-mentioned plants. The result was the same with Exp. III. but the drops were larger, and none were discernible either on the upper or under surfaces.

VI. Exposed a square of glass, some pieces of cloth, wool, dry wood, &c. on the top of a building, about 60 feet from the ground; all which in the morning, were very copously wetted on their upper surfaces, but not underneath.

From these experiments, particularly the 2nd and 6th, and part of the 3d, it appears, that some part of the dew actually falls; and, from the 1st, 4th, and 5th, and part of the 2d and 3d, that no small quantity of it rises; that is, perspires. It appears also from the 4th, that it rises by perspiration from the plants themselves, for if it had risen in vapour from the earth, it would have been found on the withered blades as well as the rest.

It seems to be a point pretty well agreed, by the naturalists, that there is a Circulation, or distribution, of the sap, or

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