To account for the comparatively low velocities of the cables, Faraday proved that they act very much as a Leyden jar acts; that is, it takes time to fill, as it were, and to discharge them, the wire coating of the cable in air acting like the outer coating of the jar or the water in the case of an immersed cable, and the retardation observed is owing to resistance of conduction, and depends upon the way in which the electrical impulses traverse the wire. “There is a long, gradual swell, and still more gradual subsidence of the electric current, and the length of time that elapses between the initial impulse and the attainment of maximum strength, is proportional to the square of the length of the line.” The duration of the electric spark has been calculated at the 1/24000 part of a second, but Professor Tyndall regards this as the longest or nearly the longest time it is perceptible; the _shortest_ time is almost inconceivable. The brightest portion of a spark has been ascertained to last only forty-six millionths of a second, and certain experiments were made to ascertain the actual duration with various numbers of Leyden jars. It was discovered by Messrs. Lucas and Cuzin, by an application of the Vernier, with batteries consisting variously of two to eight jars, and obtained the following results[14]— Duration in No. of Jars. millionths of a Second. 2 26 4 41 6 45 8 47 “So,” adds the writer, “the duration (of spark) increases in proportion with the number of the jars. It increases also with the striking distance, but is independent of the diameter of the balls or globes between which the spark strikes.” Many examples might be given of the _spark discharge_ of the electric current. This form is seen in the blue line extending between the knob of a machine and the hand, and the duration of a spark with a jar charged with an induction coil is stated by Professor Rood to vary, but the brightest portion with a jar of 114 square inches only existed for the 175th _billionth_ of a second, and with a smaller surface was much shorter. Such a spark may be conducted to a plate of gunpowder and will not ignite it, because the time of the duration of the “fire” is not sufficiently long; the powder will be scattered, but not ignited. If, however, a partly non-conducting medium be interposed between the jar and the powder, so that the spark be retarded a little, the gunpowder will be fired. While speaking of electric discharge we may remark upon the beautiful effect of lightning. These discharges are sometimes miles long, and by the return stroke from the cloud may kill a person a long way from the actual discharge. This phenomenon was illustrated by Viscount Mahon in 1779, in a very interesting book on the principles of electricity.