aid gratuitously rendered to the poor. Dr. Fothergill exerted himself in various ways to avert the contest between Great Britain and the American colonies, and he was authorized by government to attempt a compromise with Benjamin Franklin before his departure from England; but the attempt proved unsuccessful. The evidence given before the house of commons by Dr. Fothergill and John Howard on gaol distempers, contributed materially to the improvement of the law on the subject of prisons. In his gardens at Upton he formed a collection of scarce and valuable plants, the most extensive and complete at that time in Europe; containing, under glass, upwards of three thousand four hundred distinct species of exotics, and in the open air, at least three thousand different species of plants and shrubs. Dr. Fothergill was the author of many essays on a variety of subjects connected with his profession, including one on the mischievous consequences of burying in towns. His works were collected and published after his decease, accompanied with a full account of his life by his friend Dr. Lettsom. He died in London in 1780.—S. F.
FOUCAUD, Jean, born at Limoges in 1747; died in 1818. Educated at a jesuit establishment, he took orders in 1789; joined violently the revolutionists; became secretary of clubs, and conducted journals. He led what would seem a gay life at cafés, and was a distinguished billiard-player. In the phraseology of the billiard-table one stroke is still called le Coup de Foucaud. Foucaud translated Fontaine's Fables into the patois of his district, and published in the same dialect, "Chansons et piéces fugitives." He published some tracts on the relations of the ecclesiastical and civil powers.—J. A., D.
* FOUCAULT, Léon, a distinguished French physicist of the present time, was born in Paris on the 18th of September, 1819, and is the son of a well-known publisher. He was educated with a view to the medical profession, but at an early period devoted himself entirely to physical science. One of his earliest discoveries was a method of preserving steadiness in the light produced by the electric discharge between charcoal points, by which he was enabled to apply that light to the photography of microscopic objects, and to purposes of illumination in general. He afterwards made, in conjunction with M. Fizeau, various series of most remarkable experiments on the properties of light and radiant heat (see Fizeau, Hippolyte Louis); amongst which may be mentioned the obtaining of light more nearly approaching to perfect homogeneity than had ever before been obtained, and thereby producing alternate bright and dark bands by the interference of rays whose difference of path amounted to about seven thousand wave-lengths, instead of seven or eight, which had formerly been about the limit. The most remarkable discoveries of M. Foucault are a method of directly measuring the velocity with which light traverses short distances in all kinds of transparent media; and two methods of demonstrating by direct experiment, the rotation of the earth. To measure the velocity of light, a small plane mirror by means of a train of wheelwork is made to spin about an axis in its own plane with great but known speed. A beam of light falls on this mirror at its middle point, and is reflected by it towards a concave spherical mirror, whose centre of concavity is at the middle point of the spinning mirror, and which therefore reflects the beam of light directly back to the middle point of the spinning mirror. If the light took absolutely no time to travel from the spinning mirror to the concave mirror and back again, it would be reflected finally back again along the course of the incident beam; but during the time occupied by the light in that transit, the spinning mirror has turned through a small angle, and the finally reflected beam is made to deviate from the original incident beam by double that angle. From the observed angle of deviation, and the known speed of revolution of the spinning mirror, the velocity of transmission of the light can be calculated, either in air, or in any transparent substance which may be interposed between the spinning mirror and the concave mirror. The velocity of light in air, as determined by this apparatus, agrees very nearly with that deduced from the aberration of the stars, and with that determined by M. Fizeau by another method, viz.—192,000 miles in a second. M. Foucault's experiments also confirm the law deduced from the wave-theory of light, that the velocities of light in different media are inversely as their refractive indices—a conclusive proof that light is a condition, not a substance; for it can be shown that if light were a substance, its velocity would be proportional directly to the refractive index of the medium.
The first method of showing the rotation of the earth depends on this principle—that if at any place on the earth's surface the rotation which that place possesses in common with the whole earth be resolved into two components, about a vertical axis and about a horizontal axis, it is the latter component only that is communicated to a pendulum free to swing in all directions at that place. The pendulum, therefore, being made to swing in a given vertical plane, that plane remains unaffected by the first component of the earth's rotation, and shows, by its change of angular position relatively to the earth in a given interval, through what angle the earth has turned; the former of those angles being to the latter as the sine of the latitude of the place to radius.
The second method of showing the rotation of the earth is by means of the instrument called the gyroscope, being a small disc, very accurate, balanced on its axis, which axis is supported on gimbals, so as to be free to turn into every possible position. The principle upon which the method depends is, that when a disturbing force is applied to a body in rapid rotation, tending to make its axis deviate; the axis does not continue to deviate to an indefinite extent from its original direction, but moves round and round a fixed line, never deviating beyond a certain limit, which is less, the less the disturbing force and the greater the momentum of rotation of the body. It is upon this very principle that the stability of the earth's own axis of rotation depends. The disturbing forces acting on the disc of the gyroscope are very small; and when it has been set spinning very rapidly on its axis, that axis continues for a long time to point in a direction which is almost absolutely fixed, and which, by its apparent change of position, shows what has been the real change of position of the earth in the contrary direction during the interval of time that has elapsed.
M. Foucault lately invented and practised with success a method of making specula for reflecting telescopes with great accuracy, and at the same time with ease and at small expense. It consists in grinding and polishing a disc of glass to the required form, and then depositing on its surface a thin layer of silver, which, being polished, forms the reflecting surface of the speculum. M. Foucault obtained in 1855, for his measurements of the velocity of light, the Copley medal of the Royal Society of London, the highest scientific honour granted in England. He is a member of the legion of honour, and physicist to the imperial observatory of Paris. His discoveries are described in papers published in the Annales de Chimie et de Physique, and in the Reports of Proceedings (Comptes Rendus) of the Academy of Sciences from 1843 to the present time.—R.
FOUCAULT, Nicolas Joseph, a learned French antiquarian, was born at Paris in 1643, and died in 1721. Son of one of the secretaries of the council of state, he was bred to the profession of a barrister, in which he rapidly obtained distinction. While serving the office of intendant in Normandy, he discovered in 1704 the ancient town of the Viducassians, near Caen, and wrote a history of the sculptures, inscriptions, and medals, long buried in its ruins. Baluze edited a MS. of Lactantius, De Mortibus Persecutorum, which Foucault had found in the celebrated abbey of Moissac.—T. J.
FOUCHÉ, Joseph, born 29th May, 1763, at Nantes, son of a captain of a merchant vessel. A delicate constitution, it is said, prevented his being brought up to follow his father's profession. He greatly distinguished himself at the college of Nantes, by his capacity, steadiness, and love of learning. At the age of nine years he entered the Institution de l'Oratoire, where he was fellow-student with Cazalès, with whom he formed the closest intimacy. Having completed his education at Paris, he afterwards gained considerable applause as a lecturer on mathematics, physics, and metaphysics, in various academies. On his marriage he settled down in his native town, and practised as an advocate, but with little success. The breaking out of the French revolution raised him into notice. He was then twenty-five years old, and was prefect of studies at Nantes. Having established a popular club at Nantes, in which he stood forth as one of its most violent orators, he was chosen by the department of the Loire Infèrieure as deputy to the national convention. He soon became attached to the violent party, and sided with Danton in voting for the death of the king. In 1793 he was sent with Collot d'Herbois to Lyons. His remarkable speech in regard to his share in the atrocities of that mission was—"The blood of crime fertilizes the soil of liberty, and
