subject of electro-magnetic engines; the idea of using electricity as a motive power being then a favorite one among scientific men. He made inventions to employ electro-magnetic force as a motor, and his first scientific paper was upon this subject. But the result of his investigations was the abandonment of any expectation of obtaining a valuable power from electro-magnetism. But, while giving up the hope of arriving at any important economical conclusions, Mr. Joule continued his researches on the laws which govern the lifting and sustaining power of the electro-magnet. Early in 1841 he gave, in the form of a lecture in the Royal Victoria Gallery, Manchester, the result of his experiments on a new class of magnetic forces, with the preliminary statement of what had been done by M. Jacobi, of St. Petersburg, and himself, in the way of applying magnetism as a motive power. Mr. Jacobi, it may be remarked, is reputed to have been the first who constructed an electro-magnetic machine capable of producing continuous movement, and which was for a long time used in impelling a boat on the Neva.[1] Mr. Joule subsequently continued this investigation in conjunction with Dr. Scoresby, and from the results of their calculations it appeared that a grain of coal consumed by a steam-engine will raise 143 pounds one foot high, while a grain of zinc consumed in a voltaic battery can raise theoretically only 80 pounds. The cost of power by electro-magnetism was estimated to be twenty-five times greater than the cost of an equal amount of steam-power. Mr. Joule had arrived at the theory of the magnetic engine when twenty-one years of age, and in 1840 he had published a paper in Sturgeon's "Annals of Electricity," demonstrating that there is "no variation in economy, whatever the arrangement of the conducting metal, or whatever the size of the battery." Kindred to the subject of electro-motive machines is that of the air-engine, to which Mr. Joule gave considerable attention.
"Dr. Joule has pursued several lines of inquiry conjointly with other philosophers. His communication to the Royal Society, 'On the Changes of Temperature produced by the Rarefaction and Condensation of Air,' in which he pointed out the dynamical cause of the principal phenomena, and described the experiments on which his conclusions were founded, led Prof. Thomson, of Glasgow, to embark with Dr. Joule in a series of elaborate investigations 'On the Thermal Effects of Fluids in Motion.' The first of their series of four papers was read before the Royal Society, in June, 1853; the last in June, 1862. The whole will be found published at length in the 'Philosophical
- ↑ Since Jacobi's elaborate experiments, many other electro-motors have been constructed. The late Mr. Sturgeon, of Manchester, with whom Mr. Joule corresponded in his early inquiries, pumped water with an electro-magnet; Mr. Davidson, of Aberdeen, drove a turning-lathe by the same power; and in 1848 Sir David Brewster sailed at the rate of a mile an hour in a boat thus impelled, and constructed by Mr. Dill win, of Swansea. In this country Messrs. Davenport and Cook investigated the subject.
