James Prescott Joule, an English natural philosopher, born at Salford, Dec. 24, 1818. At the age of 15 he became the pupil of Dr. John Dalton, the author of the atomic theory, who trained him in the art of physical experimentation and the philosophy of chemistry, and taught him mathematics. His first scientific paper was upon the construction of electro-magnetic engines; but on account of the difficulties in the way, the chief of which is the rapid decrease of attraction accompanying increase of distance between magnets, he soon relinquished the design of producing a practical motor. In 1841 he gave a lecture in the royal Victoria gallery at Manchester on the results of his experiments on a new class of magnetic forces, which embraced a statement of what had been done by Jacobi of St. Petersburg and himself in applying magnetism as a motive power. Continuing the investigation in connection with Mr. Scoresby, Joule arrived at the result that a grain of coal consumed by a steam engine will raise 143 lbs. one foot in height, while a grain of zinc consumed in a voltaic battery can only raise, theoretically, a weight of 80 lbs. through the same distance; and that the cost of power by electro-magnetism is about 25 times greater than that of steam.

His communication to the royal society " On the Change of Temperature produced by the Rarefaction and Condensation of Air" led Prof. Thomson of Glasgow to unite with him in investigating the thermal effects of fluids in motion. The first of the series of papers on this subject was read before the royal society in June, 1853, the last in June, 1862; and they were all published in the " Philosophical Transactions." He also published, in connection with Dr. Lyon Play-fair, an account of investigations into the volumes occupied by bodies when in a solid state, and when dissolved in water; a subject having many important relations to molecular physics. His inventive talent was early displayed in the construction of galvanometers, the use of which was so constantly required in his electro-magnetic investigations. In 1863 he described to the Manchester society a new and sensitive thermometer, with which he was enabled to detect heat in the moon's rays. The principal subject to which he has devoted himself, however, is that of heat in its relation to mechanical power.

His labors in this direction commenced about the year 1840, when he communicated to the royal society the discovery of a principle in the development of heat by voltaic action, in which he established certain relations between heat and chemical affinity. The experiments of Count Rumford in 1796-'8 had exposed the fallacy of the caloric or material theory of heat, and had very nearly established the mechanical equivalent of heat, and Prof. Mayer of Heilbronn had announced his belief that the heat evolved in compressing a gas was exactly equal to the compressing force; but these views required for their complete establishment the demonstration by experiment. Placing water in a vessel made for the purpose, Joule agitated it by paddles driven by a measured force, and determined both the amount of heat produced by stirring the liquid, and the amount of labor expended. He also measured the amount of heat produced by revolving cast-iron wheels against one another. He varied the experiments by forcing water through capillary tubes, and calculating the heat generated by the friction produced.

He employed other liquids in place of water, such as oil and mercury, and although he found a different degree of sensible heat evolved with the same force expended upon different fluids, still he found that it was exactly in the inverse proportion of the fluid's specific heat, thus adding another proof of the correctness of his opinions, and of his methods of experimenting. By numerous trials he found that the quantity of heat required to raise one pound of water one degree F. in temperature is precisely competent to raise 772 pounds avoirdupois one foot in height, or in other words, is equal to 772 "foot pounds," which is the measure of the force called the mechanical equivalent of heat. (See Correlation of Forces.) In consideration of these important labors, the royal medal of the royal society was awarded to him in 1852, and in 1860 he received the Copley medal. His contributions to. scientific periodicals and other publications have been numerous and important. He was elected a fellow of the royal society in 1850; has received the degree of D. C. L. from Oxford, and of LL. D. from Dublin and Edinburgh; is a corresponding member of the institute of France; and was president of the British association for the advancement of science in 1873.