1 magnet be brought under the vessel, opposite to the end of either wire, a rapid rotation of the mercury takes place in this or that direction, according to the pole of the magnet employed; or according to the direction in which the current of electricity is transmitted. 'Masses of mercury of several inches in diameter were set in motion, and made to revolve in this manner, whenever the pole of the magnet was held near the perpendicular of the wire; but when the pole was held above the mercury between the two wires, the circular motion ceased; and currents took place in the mercury in opposite directions, one to the right, and the other to the left of the magnet. These circumstances, and various others which it would be tedious to detail, induced me to believe that the passage of the electricity through the mercury produced motions independent of the action of the magnet; and that the appearances which I have described were owing to a composition of forces. I endeavoured to ascertain the existence of these motions in the mercury, by covering its surface with weak acids; and diffusing over it finely divided matter, such as the seeds of lycopodium, white oxide of mercury, &c. but without any distinct result. It then occurred to me, that from the position of the wires, currents, if they existed, must occur chiefly in the lower, and not the upper surface of the mercury; and I consequently inverted the form of the experiment. I had two copper wires, of about one-sixth of an inch in diameter, the extremities of which were flat and carefully polished, passed through two holes three inches apart in the bottom of a glass basin, and perpendicular to it; they were cemented into the basin, and made non-conductors by sealingwax, except at their polished ends; the basin was then filled with mercury, which stood about a tenth or twelfth of an inch above the wires. The wires were now placed in a powerful voltaic circuit. The moment the contacts were made, the phenomenon, which is the principal object of this paper, occurred: the mercury was immediately seen in violent agitation; its surface became elevated into a small cone above each of the wires; waves flowed off in all directions from these cones; and the only point of rest was apparently where they met in the centre of the mercury between the two wires. On holding the pole of a powerful barmagnet at a considerable distance (some inches) above one of the cones, its apex was diminished and its base extended: by lowering the pole further, these effects were still further increased, and the undulations were feebler. At a smaller distance the surface of the mercury became plane; and rotation slowly began round the wire. As the magnet approached, the rotation became more rapid, and when it was about half an inch above the mercury, a great depression of it was observed above the wire, and a vortex, which reached almost to the surface of the wire. In the first experiments which I made, the conical elevations or fountains of mercury were about the tenth or twelfth of an inch high, and the vortices apparently as low; but in the experiments made at the London Institution, the mercury being much higher above the wire, the elevations and depressions were much more considerable, amounting to the fifth or sixth of an inch. Of course, the rotation took place with either pole of a magnet or either wire, or both together, according to the well known circumstances which determine these effects.' It is difficult to reconcile the phænomenon above described with any of the hypotheses yet advanced to explain electromagnetic action; unless, indeed, it should be found consistent with that which assumes the transmission of two fluids, one from each extremity of the machine, and which attributes the magnetic effects produced to the conflict of the electricities. An Account of an Apparatus on a peculiar Construction for performing Electro-Magnetic Experiments. By W. H. Pepys, Esq.-The reader must not understand, by the expression ⚫ peculiar construction,' that this machine is entirely on a novel principle; for it is merely in its magnitude that it is peculiar, although the form of it is by no means common in this country. It consists of two plates, each fifty feet in length, and two feet in width; the one copper, aud the other zinc, making a superficial surface of four hundred feet. They are rolled or wrapped round a cylinder of wood with three strands or ropes of horse hair between each plate, to prevent contact of the metals; and to main-` tain these in their situation, notched sticks are occasionally introduced in the rolling. Two conductors of copper near three-fourths of an inch in thickness are secured to the end of each plate, from which the power is dispensed upon immersion in the acid. To allow of the free use of so bulky an instrument, it is suspended by ropes and pullies, with a counterpoise weight, to allow its immersion in a tub of dilute acid, or when not in use, in one of water; it requires about fifty-five gallons of fluid, and the strength of the solution used has been about one-fortieth of strong nitrous acid. Upon immersing the instrument in the dilute acid, and uniting the two conductors, magnetic needles on their stands were very sensibly affected for five feet from the conductors. Cylindrical bars of steel placed in the interior of a glass tube, surrounded by a spiral of wire, and forming part of the circuit, were made powerfully magnetic, so as to be suspended from each other. When the tube and spiral were placed perpendicularly, steel cylinders or bars inserted were supported entirely by the attraction; one of these cylinders weighing 272 grains; when the contact was broken, the cylinder fell from its gravity, but instantly rushed into its former place upon the contact being made. The copper plate conductor gave the north magnetic pole, and the zinc plate conductor gave the south magnetic pole. This apparatus, as might be expected, has no intensity as a chemical agent, not even giving a spark with charcoal. But an extraordinary proof of its low intensity is, that leaves or lamina of the metals are not deflagrated, and very small portions of wire are ignited.' We have seen this machine in action, and would beg to suggest two alterations, which we are convinced would be found improvements: the one is to raise and lower the battery by a windlass instead of the counter-weight; and the other, to defend the very weak edges of the plates by wires of the same metal as themselves. Letter from Captain Basil Hall, R.N., to Captain Kater, communicating the Details of Experiments made by him and Mr. Henry Foster, with an Invariable Pendulum; in London; at the Galapagos Islands in the Pacific Ocean, near the Equator; at San Blas de California, on the North-west Coast of Mexico; and at Rio de Janeiro in Brazil. With an Appendix, containing the Second Series of Experiments in London, on the Return. An Account of Experiments made with an Invariable Pendulum at New South Wales. By Major-General Sir Thomas Brisbane, K. C. B. The nature of these experiments is now so well understood, that it would be superfluous to attempt any description of them, or of the observations on which they depend. We shall therefore content ourselves with copying the results only, shewing the various ellipticities, as deduced from the comparison of different experiments with each other. In this table, the results are obtained by the observations of Captain Hall, and they do not appear to have been repeated at this place by his very able coadjutor, Lieutenant Foster: but, in the next two places, we have the independent observations of both these very accurate inquirers. Ellipticities deduced from Captain Hall's Observations at San Blas. |Diminution of Stations compared with San Blas, in Lat. Gravity_from Ellipti 21° 32′ 24′′ N. Pole to Equa- city. tor. Length of Ellipticities deduced from Lieut. Foster's Observations at San Blas. Diminution of Stations compared with San Blas, in Lat. Gravity from Ellipti 21° 32′ 24′′ N. Pole to Equa- city. tor. Length of the |