GENUS IV. VIPERA. Daud.

Sp. 4. V. communis, Flem. Brit. Anim. p. 156.

COMMON VIPER.-Very rarely met with in Cambridgeshire. It has never occurred to my certain knowledge at Bottisham; but I am informed by Dr. Haviland, that some years back a man was brought to Addenbrooke's Hospital, who had been bitten by a venomous snake at the back of Queen's College, which was supposed to have been of this species. It has also been met with in two other instances in the neighbourhood of Cambridge.

ORDER III. BATRACHIA.

GENUS V. TRITON. Lauren.

Sp. 5. T. palustris, Flem. Brit. Anim. p. 157.

WARTY EFT.-This species, which is very common in all our ditches in the spring months, may be often observed at other periods of the year, under stones and rubbish in damp places, in a state of quiescence. No doubt, in some instances, this is in consequence of the drying up of the waters in its accustomed haunts from the heat of the summer; but in others, it appears to be the result of choice. It is also occasionally found in cellars.

Sp. 6. T. aquaticus, Flem. Brit. Anim. p. 158.

WATER EFT.-Smaller than the last species, from which it is easily distinguished by its comparatively smooth skin. Common in stagnant waters.

Sp. 7. T. vulgaris, Flem. Brit. Anim. p. 158.

COMMON OF LAND EFT.-This species is met with under stones, and in cellars; but, as far as my observation goes, is never found Vol. III. Part II.

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in the water. I agree with the Rev. Revelt Sheppard, (Linn. Trans. Vol. VII. p. 55.) in believing that it undergoes no change, and that it is perfectly distinct from either of the foregoing species.

GENUS VI. RANA. Linn.

Sp. 8. R. temporaria, Flem. Brit. Anim. p. 158.

COMMON FROG.-This well-known reptile spawns about the middle of March, and the young tadpoles are hatched a month or five weeks afterwards, according to the warmth of the season. By the eighteenth of June, I have observed these to be nearly full-sized, and beginning to acquire their fore-feet and towards the end of that month or the beginning of the next, (varying in different years,) the young frogs may be seen in great numbers, forsaking the water in which they were bred, and coming on land.

GENUS VII. BUFO. Cuv.

Sp. 9. B. vulgaris, Flem. Brit. Anim. p. 159.

COMMON TOAD.-Where this and the preceding species spend the winter does not appear to have been satisfactorily ascertained. It is, however, a curious circumstance, that from the end of February to the beginning or middle of April, they are to be found in countless numbers at the bottoms of all our ditches, ponds, and other stagnant waters. During this interval, which is the period of the breeding season, they keep up a perpetual croaking, and the act of copulation, which lasts several days, is performed. From many years' observations, I find that the Toad is invariably a few days later in spawning than the Frog. In some seasons, this difference has amounted to more than a fortnight.

Sp. 10. B. Rubeta, Flem. Brit. Anim. p. 159.

Natter-Jack.—-Found, as stated in the former part of this paper, on Gamlingay Heath, at Bottisham, and also in the Botanic Garden at Cambridge.

From the small size of the individuals found at Gamlingay, of which some appeared to have not very long left the water and passed into a perfect state, together with the lateness of the season when they were observed,-I conclude that this species does not spawn so early in the year as either the common Toad or Frog.

SWAFFHAM BULBECK,

Feb. 20, 1830.

L. JENYNS.

XVIII. On the General Equations of the Motion of Fluids, both Incompressible and Compressible; and on the Pressure of Fluids in Motion.

BY J. CHALLIS, M.A.

FELLOW OF TRINITY COLLEGE, AND OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY.

[Read Feb. 22, 1830.]

I. Incompressible Fluids.

1. THE general equations relating to the motion of incompressible fluids, are,

(Poisson, Traitè de Mecanique, Tom. II. p. 486.)

p is the pressure at any point, the co-ordinates of which are x, y, z; u, v, w, are the velocities in the directions of x, y, z, respectively; dV is put for Xdx+ Ydy + Zdz, in which X, Y, Z, are the components in the directions of x, y, z, respectively, of Vol. III. Part III.

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