ATMOSPHERIC by passes from 5000 to 6000 feet high. Both slopes arewooded, and its forests are the only parts of Morocco where the lion still survives. The Little or Anti-Atlas running parallel to and south of the main range is one of the least elevated chains in the system, having a mean altitude of not more than 5000 feet, although some peaks and even passes exceed 6000 feet. At one point it is pierced by a gap scarcely five paces wide with polished walls of variegated marbles. Although in some winters nearly free from snow, La Martini ere found the Little Atlas quite covered with a white mantle in 1890-91. The Atlas system is completed by two other less important ranges — the Jebel-Bani, ATMOSPHERIC § 1. Investigation into the early history of atmospheric electricity would be the province rather of the antiquary than of the scientific man. In pre-scientific days lightning was usually regarded as the special vehicle of divine vengeance. Lucretius was perhaps the first who attempted to apply to its elucidation the scientific method; he pointed out that even temples of Jupiter had not escaped the thunderbolt, which seemed to imply an incredible degree of inefficiency in the department over which that divinity was supposed to preside. Like most modern scientific men who have considered the subject, Lucretius had a theory of his own, and he might almost be looked upon as the parent of the “ electric fluid ” phraseology still so prominent in popular writings. Even in 1749, when Franklin proposed the famous kite experiment to test the identity of natural and artificial electricity, the electric fluid theory was probably held by most scientific men in a fairly literal sense. The electric fluid was supposed to reside in the clouds, whence it was attracted by any sharp point held upwards. Shortly after 1750 the observations by Lemonnier and others of electric phenomena in the atmosphere in the absence of cloud led to some modification of ideas. It was only, however, with the introduction of improved methods and apparatus after the middle of the 19th century, mainly by Lord Kelvin, that there was any approach to exact measurements of atmospheric electricity. To understand the different methods employed or recommended, a bifief survey of the phenomena is necessary. § 2. At most places in dry weather the electric potential near the ground is normally positive relative to the earth, and increases with the height. The existence of earth currents shows that the earth, strictly speaking, may not be all at one potential; but the potential differences between points on the earth’s surface—say, 1 kilometre apart—are insignificant compared to the normal potential difference between the earth and a point 1 metre above it, and when considering this latter difference we may regard the earth as at a uniform zero potential. What is aimed at in ordinary observations of atmospheric electricity is the measurement of the potential difference between the earth and a point at a given height above it, or else the difference in potential between two points in the same vertical at a given distance apart. Let a conductor—say, a metallic sphere—be supported by a metal rod of negligible capacity whose other end is earthed. Then the conductor must all be at meats' zer0 P0tential, and consequently the sphere must have an induced charge which produces at its centre a potential equal, but opposite, to what would exist at the same spot in free air. This neglects any charge carried by the air displaced by the sphere, and assumes a statical state of conditions, and further, what in practice is
ELECTRICITY
773
south of the Little Atlas, crossed several times by Foucauld, who describes it as a low, narrow rocky ridge with a height of 3000 feet in its central parts; and the Ulmess Riata, south of the middle range, not a continuous range, but a series of broken mountain masses from 3000 to 3500 feet high, and traversed by the rivers Sebu and Muluya. Authorities.—Thomson, Joseph. Travels in the Atlas and Southern Morocco. London, 1889.—Lenz, Dr Oscar. Timbuktu. Leipzig, 1884.—De Foucauld, Vicomte Ch. Reconnaissance au Maroc, 1883-84, Paris, 1888. — MartiniAre, H. M. P. de la. Morocco; Journeys to the Kingdom of Fez, &c. London, 1889.— Harris, Walter B. Tafilet, a Journey of Exploration in the Atlas Mountains, &c. London, 1895. (a. H. K.) ELECTRICITY. never wholly realized, that the conductor exerts no disturbing influence outside itself. Suppose now that the sphere’s earth connexion is broken, and that it is carried by an insulating handle inside a building at zero potential, where its potential (relative to the earth) is determined. If this potential is - V (volts), then, assuming no loss of electricity during the operations, the potential of the air at the spot occupied by the sphere was + V. This method under various forms was largely in use about the middle of the 19th century. It was followed for many years by Quetelet at Brussels, and in a modified form by Palmieri at Naples ; in recent years it has been employed at the suggestion of Elster and Geitel on the top of the Sonnblick. Next, suppose that a fixed conductor is insulated from the ground, and that by some means it is kept at the potential of the air which it displaces, then the measurement of its potential is equivalent to a measurement of the potential of the air. This is the basis of various seemingly different methods. Originally the conductor took the shape of long metal wires, supported by silk or other insulating material, and no artificial means were employed to bring them to the potential of the surrounding air. The addition of sharp points was a step in advance. But the method can hardly be said to have been a quantitative one until the sharp points were replaced by either a flame (fuse, gas, lamp) or a liquid jet breaking into drops. In either case the theory is the same. The matter leaving the conductor, whether it be the products of combustion of a fuse or the drops of a liquid, supplies the means of maintaining equality of potential between the conductor and the air at the spot where the matter quits electrical connexion with the conductor. It is customary to apply the term collector to the flame or liquid apparatus, its function being, in popular language, to collect electricity. § 3. Several forms of flame-collector have been proposed, and two- have been a*good deal used, viz., Lord Kelvin’s portable electrometer {Papers on Electrostatics and Magnetism, §§ 263, 277, &c.) with a fuse, and Exner’s form of gold-leaf electroscope (Exner, JVien. Sitz. vol. xciii. (Ab. ii.), 1886, p. 222 ; also Sitz. xcv., xcviii., xcix., &c. ; Elster and Geitel, Terrestrial Magnetism, vol. iv. p. 15, <fec.) in conjunction with an oil lamp or gas flame. The latter is the instrument employed in most of the recent wTork done by Exner, Elster and Geitel, and others in Austria and Germany. The most obvious defect of the ordinary flamecollector is the tendency in the flame to be blown out. This prevents the use of Exner’s form on windy days, but Pellat {Soc. Franc. Phys. Bull. June 1899; Science Abstracts, 1706, for 1899) has invented a form which claims to have surmounted this difficulty. Of liquid collectors the representative is Lord Kelvin’s “water-dropper.” The water is contained in a tank standing on insulated supports, and the tube through which the jet
