io6 THE POPULAR SCIENCE MONTHLY, 



by-tlie-way, at whicli 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 Me- 

 chanical Equivalent of Heat,' detailed the results he had gained from 

 water agitated by a paddle-wheel. In following years, the same sub- 

 ject was perseveringly j^rosecuted, 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 quan- 

 tity of force expended ; ' 2. * The quantity of heat capable of increasing 

 the temperature of a pound of water by 1° Fahr., requires for its evo- 

 lution the expenditure of a mechanical force required by the fall of 

 '^72 pounds through the space of one foot.' " 



Dr. Tyndall gives the following explanation of the term " foot- 

 pounds," 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 tho 

 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-jDOunds 

 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 establish- 

 ment of the truth of the mechanical equivalent of heat. Dr. Joule 

 states his own relation to the investigation as folloAvs : "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 pub- 

 lish 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." 



