by breaking connection with the conductor which carries away its current, becomes, when shown in a darkened room, clearly visible. In "Electricity in the Service of Man" it is said, page 63, "The inductive action of the machine is quite as rapid and as powerful when both collectors are removed and nothing is left but the two rotating disks and their respective contact or neutralizing brushes. The whole apparatus then bristles with electricity, and if viewed in the dark presents a most beautiful appearance, being literally bathed with luminous brush discharges." This is a true aurora. Let us now examine some of these more recent electric machines,--the later induction, not the older frictional machines, for it is obvious that the rotation of the planets, if they operate as electric generators, or separators, must act by induction and not by friction. The frictional machines are of the old type and are well known from the books; in these a glass disk or cylinder is rubbed upon in its rotation by an amalgamated (so called) friction pad fixed securely to the bed of the machine. But more recently these have been replaced by far more powerful and simple machines which operate entirely by induction, like approaching thunderclouds, for instance, and in which one or more glass disks are merely rotated rapidly and freely in the air, these disks having a number of light metallic sectors, such as bits of tin-foil, pasted on their outer sides at equal radial intervals, and with metallic collecting brushes which, however, barely graze the surfaces of the rotating disk. There is no pressure and no friction, except that of the disks as they freely revolve in the atmosphere.