Mr. J. P. Joule found that the economical duty of an electro-magnetic engine at a given velocity and for a given resistance of the battery is proportioned to the mean intensity of the several pairs of the battery. With his apparatus, every pound of zinc consumed in a Grove's battery produced a mechanical force (friction included) equal to raise a weight of 331,400 pounds to the height of one foot, when the revolving magnets were moving at the velocity of eight feet per second. Now, the _duty_ of the best Cornish steam-engine is about one million five hundred thousand pounds raised to the height of one foot by the combustion of each pound of coal, or nearly five times the extreme _duty_ that could be obtained from an electro-magnetic engine by the consumption of one pound of zinc. This comparison is therefore so very unfavourable, that the idea of a successful application of electricity as an _economic_ source of power, is almost, if not entirely abandoned. By instituting a comparison between the different means of producing power, it has been shown that for every shilling expended there might be raised by Pounds. Manual power 600,000 one foot high in a day. Horse 3,600,000 " " Steam 56,000,000 " " Electro-magnetism 900,000 " " A powerful magnet has been compared to a steam-engine with an enormous piston but with an exceedingly short stroke. Although motive power cannot be produced from electricity and applied successfully to commercial purposes, like the steam-engine, yet the achievements of the electric telegraph as an application of a small motive power must not be lost sight of, whilst the fall of the ball at Deal and other places, by which the chronometers of the mercantile navy are regulated, as also the means of regulating the time at the General Post Office and various railway stations, are all useful applications of the power which fails to compete in other ways with steam. [Page 218] CHAPTER XVII. THE ELECTRIC TELEGRAPH. The engineering and philosophical details of this important instrument have grown to such formidable dimensions, that any attempt (short of devoting the whole of these pages to the subject) to give a full account of the history and application of the instrument, the failures and successes of novel inventions, and the continued onward progress of this mode of communication, must be regarded as simply impossible, and therefore a very brief account of the _principle_ only will be attempted in these pages.