Page:Popular Science Monthly Volume 35.djvu/531

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ures." Offering-stones, with little cup-shaped holes, are sometimes found on the roof-stones of graves of the Stone age. They are now popularly called "elf-mills" and are still regarded as holy; and, it is said, offerings are still secretly made in them.

That the Stone age lasted for a very long time in the North is proved, among other things, by the fact that this period reached a far higher development there than anywhere else in Europe. At what time it began in Sweden we can not even approximately determine; but everything seems to show that it ended rather before than after 1500 B.C., and, therefore, about three thousand five hundred years before our time. In many countries of the east and in the south of Europe the Stone age came to an end long ago; while in some parts of the New World this stage of civilization has continued to our own day.


SINCE the time when Maxwell occupied himself with the theory of electricity, perhaps even since the time of Faraday, it has been generally accepted by most physicists that electricity is a phenomenon resulting from oscillations of the luminiferous ether. However, with the exception of a few experiments on inductive capacities, etc., instigated by Maxwell's "electro-magnetic theory of light," no direct experimental verifications of this hypothesis had been made until the latter part of last year, when Prof. Hertz, of Carlsruhe, Germany, commenced a series of experiments on the interference of electrical waves. In all, six articles have been published—two in Band 31 and four in Band 34 of the "Annalen der Physik und Chemie." The earlier articles are of a qualitative character, while the later are quantitative. The former are of less interest than the latter, because the phenomena are less striking and are not so decisive as a proof. They are substantially as follow: The secondary electrodes of a large Ruhmkorff coil consist of two brass rods whose ends are surmounted with brass balls. The two rods are in the same straight line, and separated from each other by a short airspace of about seven millimetres in length. This is the general form of discharger in a Ruhmkorff. From either of these electrodes is led a wire, which connects with a rectangularly bent wire, which, however, is not completely closed, but is cut in some portion, and each of its ends surmounted by brass balls.

  1. Read before the Mathematical Physical Club of Boston and Cambridge, December 17, 1888.