The velocity of electricity is very great, and experiments have frequently been made. Wheatstone undertook to ascertain the speed of the electric fluid, and the instrument he employed he called a “Chronoscope.” He caused a mirror to revolve with enormous velocity, and measured the speed by the vibrations of air, which produced a certain note by the same motive power. (We know already that certain notes are produced by a certain number of vibrations per second.) Wheatstone’s Chronoscope consisted of this mirror, in front of which was placed a circular block of wood, in which, in a row, were set six wires carrying small knobs; round these and over the wood he put an insulating varnish. A Leyden jar was connected outside with the first knob; between the second and third a quarter of a mile of copper wire was coiled, and a like length of wire between the fourth and fifth; the inside of the Leyden jar was then connected with the last knob, and the spark passed; it ran from knob to knob over the long coils of wire. If all the flashes over the wire and knobs occurred simultaneously the mirror would show them side by side; if not, as the mirror turned a trifle, the difference would be observed. The mirror did show a slight retardation in the passage of the flash, and from certain measurements and calculations Wheatstone estimated the velocity of the spark to be 288,000 miles a second. This rate will carry electricity round the earth in about a twelfth of a second, a rate Puck never dreamed of when he promised to “put a girdle round the earth in forty minutes.” But it appeared from investigations subsequently made that it was not possible to express the velocity of electricity with any certainty, and a number of experiments were made as per the following table, with very different results. Sir William Thomson and Faraday endeavoured to account for these stupendous discrepancies, and the principles of retardation of electricity were established. The differences are shown below:— Nature of Velocity per Wire. Second. Wheatstone’s Experiment in 1834 Copper 288,000 Gonella and Fizeau {Copper 111,834 {Iron 62,130 Mitchell Iron 28,331 Walker Iron 18,639 Gould Iron 15,830 Astronomers in Greenwich Copper 7,600 “ Brussels Copper 2,700 Result in Atlantic Cable, 1857 Copper 1,430 “ “ 1858 Copper 3,000