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106 by-the-way, at which Dalton appeared; and, on August 21, 1843, a circumstance which requires special mention, he communicated a second paper to the Association, then meeting at Cork, in which he describes a series of experiments on magneto-electricity, executed with a view to determine the mechanical value of heat. Experiments, with a like object, on the condensation of air, were communicated to the Association in 1844; and in 1845 his important paper, 'On the Mechanical Equivalent of Heat,' detailed the results he had gained from water agitated by a paddle-wheel. In following years, the same subject was perseveringly prosecuted, by numerous and yet more accurate experiments, until his grand determination was finally reached. In an elaborate paper, read before the Royal Society, January 21, 1849, and published in the 'Philosophical Transactions' of 1850, we have the results thus stated: 1. 'The quantity of heat produced by the friction of bodies, whether solid or liquid, is always proportional to the quantity of force expended;' 2. 'The quantity of heat capable of increasing the temperature of a pound of water by 1° Fahr., requires for its evolution the expenditure of a mechanical force required by the fall of 772 pounds through the space of one foot.' "

Dr. Tyndall gives the following explanation of the term "footpounds," used as a measure by Joule: "The quantity of heat which would raise one pound of water one degree in temperature is exactly equal to what would be generated if a pound-weight, after having fallen 772 feet, had its moving force destroyed by collision with the earth. Conversely, the amount of heat necessary to raise a pound of water one degree would, if applied mechanically, be competent to raise a pound-weight 772 feet high, or it would raise 772 pounds one foot high. The term 'foot-pound' expresses the lifting of one pound to the height of a foot. Thus the heat required to raise the temperature of one pound of water one degree being taken as a standard, 772 foot-pounds constitute what is called the mechanical equivalent of heat."

A sharp controversy arose a few years since in England as to the relative merits of Mayer and Joule in contributing to the establishment of the truth of the mechanical equivalent of heat. Dr. Joule states his own relation to the investigation as follows: "Mayer," he says, "appears to have published his views for the express purpose of securing priority. He did not wait until he had the opportunity of supporting them by facts. My course, on the contrary, was to publish only such theories as I had established by experiments calculated to commend them to the scientific public, being well convinced of the truth of Sir John Herschel's remark, that 'hasty generalization is the bane of science.' . . . I therefore fearlessly assert," writes Dr. Joule, in August, 1862, "my right to the position which has been generally accorded to me by my fellow-physicists, as having been the first to give decisive proof of the correctness of this theory."