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l as in case of fire, etc. A depth of rather more than 2,700 feet was penetrated, mostly, if not entirely, through Silurian strata, but none was reached where the water had a sufficient head to rise to the surface. Artesian wells were made in ancient times in the Oasis of El-Bacharich, and were described by Olympiodorus, a native of Thebes, who lived in the fifth century A. D. Their depth is said to be from 200 to 500 cubits, and the water issues at the surface. They have been noticed by Arago. A Frenchman has reopened several of those which had become stopped. The reopened wells are from 360 to 480 feet deep. The Moniteur Algerien gives an interesting report on the newly bored Artesian wells in the Sahara Desert, in the province of Constantine. The first well was bored in the Oasis of Oued-Rir, near Tamerna, by a detachment of the Foreign Legion, conducted by the engineer, M. Jus. The works were begun in May, 1856, and, on the 19th of June, a quantity of water, of 1,060 ga
the dichroism of crystals consists in transmitting different colors when viewed in different directions. There are several varieties of this apparatus invented by Arago and Brewster. As constructed by Brewster, it consists of a tube about two inches long, blackened on the interior, and attached to a ball and socket. The ballded by a series of glass rings of a spherical triangular form, so as to produce the same effect upon the rays as is produced by the central lens. Allan Stevenson, Arago, and Faraday are credited with improvements in the details. Fresnel's dioptric light. The flame is placed in the focus of the lenses, and the beams are bent discharge of such liquid waste from manufactories as would foul the running water of streams. Such wells are properly termed absorbing-wells (which see), and by Arago are called negative artesian-wells, — a term more curious than profound. In former times the plain of Paluns, near Marseilles, was a morass, but was drained by me
manner. Galva-nom′e-ter. An instrument for measuring the strength of magnetic currents. The instrument as usually constructed consists of a magnetized needle placed parallel to a wire, which, when electrically excited, causes the deflection of the needle. See electrometer; electroscope. The discovery of this property in an electric current was by Oersted of Denmark, in 1819. The principle was soon adopted by electricians in the construction of the indicator telegraph. Ampere, Arago, Schilling, Gauss, Weber, and Alexander all used the principle, but it received its perfected form by Cooke and Wheatstone, English patent, 1837. See indicator-telegraph. Galvanometers. The tendency of the magnetic needle in the vicinity of an electrically excited needle held parallel to it is to assume a position at right angles to the wire conveying the current. By making the needle astatic, that is, by placing two needles with their poles in opposition to each other, they are no
carried on the person, though it has been often suggested. The risk is so small, numerically considered, that it is not probable any great proportion of the inhabitants of any country will make special provision for avoiding the danger. Professor Arago classed several well-known sites according to the frequency of their storms, from the best information he could obtain. His list begins as follows: — Days of Thunder per Year. 1. Calcutta averages60 2. Patna (India) supposed to averay mixtures containing phosphorus. See lighting-devices. Lu-cim′e-ter. An instrument for measuring the intensity of light. A photometer. Various photometers have been invented by Huyghens, Rumford, Celsius, Bouguer, Priestley, Ritchie, Arago, and others, which are described under that head. See photometer. Celsius called his device a lucimeter, and the name has since been adopted by Deslaurier in his modification of Bouguer's apparatus. Deslaurier's apparatus is designed for
city from the atmosphere over them. The intention is to prevent the occurrence of hail-storms. Arago proposed that the conductors should be raised and supported by small balloons connected by slendtion; it may also be produced by the passage of light through rock crystal, as was discovered by Arago, or through some fluids, as oil of turpentine or solution of sugar, as was ascertained by Biot apsed before the discovery of this isolated phenomenon was followed by the discoveries of Malus, Arago, Fresnel, Brewster, and Biot. Malus, in 1808, discovered polarization by reflection from polished surfaces; and Arago, in 1811, discovered colored polarization. A world of wonders of variously modified waves of light gifted with new properties was now opened. A ray of light which reaches ourproperties of our atmosphere, and the position of the four neutral points of polarization, which Arago, Babinet, and Brewster discovered. Thus man makes for himself as it were new organs which, when
them. One instrument is the binocular photographic camera, and the other instrument is the stereoscope. The following story is told of the first introduction of the stereoscope to the sarants of France. The Abbe Moigno took the instrument to Arago, and tried to interest him in it; but Arago unluckily had a defect of vision which made him see double, so that on looking into the stereoscope he saw only a medley of four pictures. The Abbe then went to Savart, but he was quite incapable of apArago unluckily had a defect of vision which made him see double, so that on looking into the stereoscope he saw only a medley of four pictures. The Abbe then went to Savart, but he was quite incapable of appreciating the thing, for he had but one eye. Becquerel was next visited, but he was nearly blind, and consequently cared little for the new optical toy. The Abbe not discouraged, called next upon Puillet, of the Conservatoire des Arts et Metiers. He was a good deal interested in the description of the apparatus, but unfortunately he squinted, and therefore could see nothing in it but a blurred mixture of images. Lastly, Biot was tried, but Biot was an earnest advocate of the corpuscular theo