ELECTRICAL WAVES. 509 



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 civili- 

 zation has continued to our own day. 



-♦♦♦- 



ELECTRICAL WAVES.* 



By SAMUEL SHELDON, Ph. D. 



SINCE the time when Maxwell occupied himself with the 

 theory of electricity, perhaps even since the time of Fara- 

 day, it has been generally accepted by most physicists that elec- 

 tricity is a phenomenon resulting from oscillations of the lumi- 

 niferous ether. However, with the exception of a few experi- 

 ments on inductive capacities, etc., instigated by Maxwell's " elec- 

 tro-magnetic theory of light," no direct experimental verifica- 

 tions 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, be- 

 cause the phenomena are less striking and are not so decisive as 

 a proof. They are substantially as follow : The secondary elec- 

 trodes 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 air- 

 space of about seven millimetres in length. This is the general 

 form of discharger in a Ruhmkorff. From either of these elec- 

 trodes 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. 



* Read before the Mathematical Physical Club of Boston and Cambridge, December 

 17, 1888. 



