In none of these extracts, however, do we find anything that has reference to the properties of the arc as a continuous, luminous spark. It was in his subsequent researches that Davy made known its properties. It will be seen, however, that the electric light had attracted attention before its special property of continuity had been observed.
It results from these facts that Robertson's experiment was in no wise anterior to that of Davy. The inventor of the phantasmagoria did not obtain the arc, properly so called, with its characteristic continuity, but merely produced a spark between two carbons--an experiment that had already been made known by Davy in 1800. The latter had then at his disposal nothing but a relatively weak pile, and it is very natural that, under such circumstances, he produced a spark without observing its properties as a light producer.
It was only in 1808 that he was in a position to operate upon a larger scale. At this epoch a group of men who were interested in the progress of science subscribed the necessary funds for the construction of a large battery designed for the laboratory of the Royal Institution. This pile was composed of 2,000 elements mounted in two hundred porcelain troughs, one of which is still to be seen at the Royal Institution. The zinc plates of these elements were each of them 32 inches square, and formed altogether a surface of 80 square meters. It was with this powerful battery that Davy, in 1810, performed the experiment on the voltaic arc before the members of the Royal Institution.
The carbons employed were rods of charcoal, and were rapidly used up in burning in the air. So in order to give longer duration to his experiment, Davy was obliged, on repeating it, to inclose the carbons in a glass globe like that used in the apparatus called the electric egg. The accompanying figure represents the experiment made under this form in the great ampitheater of the Royal Institution at London.--La Lumiere Electrique.
ELECTRICAL GRAPNEL FOR SUBMARINE CABLES AND TORPEDO LINES.
By H. KINGSFORD.
All those who are acquainted with the cable-lifting branch of submarine telegraphy are well aware how important a matter it is in grappling to be certain of the instant the cable is hooked. This importance increases, of course, with the age and consequent weakness of the material, as the injury caused by dragging a cable along the bottom is obviously very great.
