So simple are great facts when finally accomplished that there remains little to add on the subject of the mechanism of the electric light. The two varieties, arc and incandescent, are used together as most convenient, the large and very brilliant arc being especially adapted to out-of-doors situations, and the gentler, steadier and more permanent glow of the incandescent to interiors. The latter is also capable of a modification not applicable to the arc. It can, in theaters and other buildings, be "turned down" to a gentle, blood-red glow. The means by which this is accomplished is ingenious and surprising, since it means that the supply of electricity over a wire--seemingly the most subtle and elusive essence on earth--may be controlled like a stream from a cock, or the gas out of a burner. But this reduction of the current that makes the red glow in the clusters in a theater is by no means the only instance. The trolley-car, and even the common motor, may be made to start very slowly, and the unseen current whose touch kills is fed to its consumer at will. [Illustration] THE DYNAMO.--To the man who has been all his life thinking of the steam engine as the highest and almost only embodiment of controlled mechanical power, another machine, both supplementary to the steam engine and far excelling it, whose familiar _burring_ sound is now heard in almost every village in the United States and has become the characteristic sound of modern civilization, must constitute a source of continual question and surprise. To be accustomed to the dynamo, to look upon it as a matter of course and a conceded fact, one must have come to years of maturity and found it here.