THE POPULAR SCIENCE MONTHLY.
in place of the cotton-cloth that had hitherto served as money, with cowries, at the rate of four thousand to the dollar, for small change. A large demand for cowries sprang up, and the trade in them was stimulated to such an excess that the market was glutted, and it afterward languished for several years. The present demand is quite lively. The cowrie-shell is used as currency principally in the countries near the Niger, except in Ashantce, where gold-dust is the medium of exchange. North of Ashantee, gold-dust and the gera or cola-nut (Sterculia acuminata) are used with cowries, a load of sixty pounds of the nuts being considered equivalent in value to about fifteen thousand cowries. The shells have been used as a medium of exchange from a high antiquity. Marco Polo found them circulating in Yunnan in the thirteenth century; and they have been discovered in prehistoric graves in the Baltic countries. Herr A. Wörmann says, in a paper of the Hamburg society, on trade by barter in Africa, that a variety of objects besides cowries serve as measures of value in the different countries of that continent. Among them are pearls, little Nuremberg looking-glasses, iron, copper, brass, cloth, salt, tobacco-leaves, writing-paper, the colanut, goats, horses, cattle, and slaves; and the regions in which each of these articles circulates are defined by fixed limits. Iron and copper from Egypt circulate in the upper Nile region; Maria-Theresa thalers and cowries in Soodan; cowries, pearls, and "Mericani" (unbleached goods) on the East coast and in the region of the Arab trade. South and west of these countries are distinct trade-regions that have no direct connection with them, in each of which a different currency is needed, although ivory and slaves are the only products.
Shooting-Stars, their Traditions and their Origin.—The appearance of comets and shooting-stars announced to the ancients and to our ancestors the death of some grand personage or some woe, and the chronicles are full of notices of such phenomena. The notices are, in fact, occasionally so numerous as to be suspicious, for, as Lubienietz remarks in his "Cometography," when an event of such a kind happened, it was thought there must have been a comet about the time, and so it was put down; and an amusing picture has been made of the perplexity of a cometographer who could not find any comet for seventeen years, portentous of the events that were to happen during that period. The Chinese records are more trustworthy, for their observers were constantly at their posts, and formed a regularly and scientifically organized body. The documents recording the observations were specially preserved; for the Chinese, from a time many centuries before the Christian era, attributed to the different stellar groups a direct influence on the different provinces of their country. As shooting-stars may be seen at almost any time, it was to be expected that a great number of notices of the phenomena must have been recorded during the forty centuries of which we have a literature of some kind. Plutarch, in his biography of Lysander, makes a near approach to the modern explanation of the origin of these bodies, saying, "Some philosophers believe that the shooting-stars are not detached parts of the ether which go out in the air soon after they have been inflamed, that they no more originate in the combustion of the air which is dissolved in great quantity in the upper regions, but that they are rather falling celestial bodies." The general opinion is, that shooting-stars are bodies of small dimensions that circulate, under the influence of attraction, among the planets in the same way as the planets themselves. When they cross our atmosphere, the friction develops heat enough to consume them, most frequently before they reach our soil. The mean height at which the meteors become luminous exceeds, however, the estimated height of our atmosphere. Poisson has, therefore, suggested that, as they could hardly have become inflamed from friction at such a height, an atmosphere of neutral electricity may exist considerably beyond the mass of the air which is subject to the earth's attraction, and is disturbed by the entrance of the meteors, so that they become electrified and incandescent. Any theory to account fully for the origin of shooting-stars must explain the periodic swarms. For this reason, the theory of ejectment from lunar volcanoes must fail, even were it not otherwise shown to be baseless. M. Faye accounts for the August meteors by