The only wheels required in a clock of this kind are those which turn the hands; and the motion is communicated from the pendulum to the seconds wheel by means of a small attached lever, working on a ratchet wheel. The minute and the hour hands derive their movements from the seconds wheel in the usual manner. The voltaic battery employed to work Mr. Bain's clocks consists of a pair of large copper and zinc plates buried in the moist earth, which excite a sufficient amount of electricity to maintain the motion of the pendulum. A battery of this kind will remain in action a long time, and will serve to keep a clock going for several months. It is, indeed, a near approach to the attainment of perpetual motion, since nothing but the wearing away of the materials, or the accumulation of dust on the connecting points, seems to prevent the realization of that mechanical chimera. There is a disadvantage attending the arrangement of Mr. Bain's clocks, arising from the attachment of the pendulum to the wheels; and as the moving force is derived directly from voltaic electricity, any variation in the power of the battery causes variation in the lengths of the vibrations, and produces irregularity. For the purpose of remedying these defects, Mr. Shepherd, junior, has adopted an arrangement which detaches the pendulum from the clock movement, and makes its vibrations altogether independent of the varying force of voltaic batteries. In Mr. Shepherd's arrangement, the impulse of the pendulum is given by successive blows from a spring, which is drawn back and then liberated at each vibration. The hands of the clock are also moved by electro-magnets, by which means the impelling forces and the resistances encountered by the pendulum are always constant. By making the pendulum thus independent of the works, and employing it merely to make and break contract at regular intervals, any number of clocks in the same establishment may be set in motion, and kept exactly together, by a single pendulum. The large clock over the principal entrance to the Great Exhibition was on this construction. It would have been impossible, with any approach to regularity, to have moved hands of that size, exposed as they were to the wind, unless the pendulum had been independent of such resistances. Electro-Magnetic Clocks have not yet come into general use, partly owing to imperfections in the battery connections, which occasionally put a stop to their movements, but principally on account of the high prices charged by the patentees. As no trains of wheels are requisite in an Electro-Magnetic Clock, it might be manufactured very cheaply; and when the price is reduced to its proper standard, and the trifling practical defects are remedied, these clocks may possibly supersede others. ELECTRO-METALLURGY.