104 THE POPULAR SCIENCE MONTHLY. 



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 emj)loy electro-magnetic force as a motor, and his 

 first scientific paper was upon this subject. But the result of his in- 

 vestigations was the abandonment of any expectation of obtaining a 

 valuable power from eleatro-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 prelimi- 

 nary 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 con- 

 tinuous movement, and which was for a long time used in impelling a 

 boat on the Neva/ Mr. Joule subsequently continued this investiga- 

 tion 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 Con- 

 densation of Air,' in which he jDointed out the dynamical cause of the 

 principal phenomena, and described the experiments on which his con- 

 clusions were founded, led Prof. Thomson, of Glasgow, to embark 

 with Dr. Joule in a series of elaborate investigations ' On the Thermal 

 Efiects 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 con- 

 structed. 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. Dillwin, of Swan- 

 sea. In this country Messrs. Davenport and Cook investigated the subject. 



