under the pole-star itself, from whence it spread both ways into the north-east and north-west."

About the time that this aurora appeared, the variation at London, the place of observation, was about 18° westward; and, consequently, neither the two steaks of light, nor the crepusculum in the north, had any relation to the magnetic meridian. The nubecula seen by Dr Halley, seems to have been of the same character as the flashes or waves of luminous vapour, seen at Prestwich during the aurora of November 21st and December 17th last; they were obviously at no very great altitude, certainly within the range of aqueous vapour; the colour of these waves was that of a dim silvery whiteness.

I perfectly agree with Halley, Hansteen, Brewster, and many other eminent philosophers, in the belief of a magnetic element or effluvium, pervading the atmosphere, and perhaps all space; but the principles of Electro-magnetism do not allow of electric currents traversing the magnetic lines of force in the direction of their length, unless constrained by other influences than any known to exist in the regions of the aurora borealis. It is possible, however, that the theoretical views which I have here advanced may be open to objections that I do not myself perceive, and may require the corrections of a more diligent observer, and a sounder reasoner on the facts observed.

On a Formula for calculating the Expansion of Liquids by Heat. By WILLIAM JOHN MACQUORN RANKINE, Esq., Civil Engineer. Communicated by the Author.

Having been lately much engaged in researches involving the comparative volumes of liquids at various temperatures, I have found the following formula very useful:

Log V represents the common logarithm of the volume of a given mass of liquid, as compared with its volume at a certain standard temperature, which, for water, is the temperature of its maximum density, or 4°1 centigrade, and for other liquids 0° centigrade.

t is the temperature measured from the absolute zero mentioned in my paper on the Elasticity of Vapours, in the Edinburgh New Philosophical Journal for July 1849, and is found by adding 274 6 to the temperature according to the centigrade scale.

A, B, and C, are three constants, depending on the nature of the liquid, whose values for the centigrade scale, corresponding to water, mercury, alcohol, and sulphuret of carbon, are given below.

The data from which the constants have been computed have been taken from the following authorities:-for water, from the experiments of Hallström ; for mercury, from those of Regnault; and for alcohol and sulphuret of carbon, from those of Gay-Lussac. As the experiments of M. Gay-Lussac give only the apparent expansion of the liquids in glass, I have assumed, in order to calculate the true expansion, that the dilatation of the glass used by him was 0000258 of its volume for each centigrade degree. This is very nearly the mean dilatation of the different kinds of glass. M. Regnault has shewn that, according to the composition and treatment of glass, the coefficient varies between the limits 000022 and ·000028.

Annexed are given tables of comparison between the results of the formula and those of experiment. The data from which the constants were calculated are marked with asterisks.

The table for water shews, that between 0° and 30° centigrade, the formula agrees closely with the experiments of Hallström, and that from 30° to 100° its results lie between those of the experiments of Gay-Lussac and Deluc.

The experiments of Gay-Lussac originally gave the appaent volume of water in glass, as compared with that at 100°.

They have been reduced to the unit of minimum volume by means of Hallström's value of the expansion between 4°1 and 30°, and the coefficient of expansion of glass already mentioned.

In the fifth column of the table of comparison for mercury it is stated which of the experimental results were taken from M. Regnault's own measurements on the curve, representing the mean results of his experiments, and which from his tables of actual experiments, distinguishing the series.

In the experimental results for alcohol and sulphuret of carbon, the respective units of volume are the volumes of those liquids at their boiling points, and the volumes given by the formula have been reduced to the same units.

On the Geographical Distribution and Uses of the Common Oyster (Ostrea edulis.)

The Ostrea edulis may be said to have its capital in Britain; for though found elsewhere on the coasts of Europe, both northwards and southwards, in no part of them does it attain such perfection as in our seas, through which it is generally distributed, sparingly in some places, abundantly, and in gregarious assemblages in others, chiefly inhabiting the laminarian and coralline zones. The ancient Romans valued our native oysters even as we do now, and must have held them in higher estimation than those of Italian shores, or they would not have brought them from so far for their luxurious feasts.

In Bishop Spratt's "History of the Royal Society," is contained the first paper of importance on the Oyster-fisheries of England. It is selected by the Bishop as one of the examples which he gives of the various kinds of papers read before the Royal Society at that time, and respecting it he well remarks, "It may, perhaps, seem a subject too mean to be particularly alleged, but to me it appears worthy to be produced. For though the British oysters have been famous in the world ever since this island was discovered, yet the skill how to order them aright has been so little considered among our