PHYSICS OF THE NINETEENTH CENT.-HEAT. 525 



have, in some measure, the stamp of a profound intuition, which rose, 

 however, to the energy of undoubting conviction in his mind. Joule's 

 labours, on the contrary, are an experimental demonstration. Mayer 

 thought out his theory and rose to its grandest applications ; Joule 

 worked out his theory and gave it the solidity of natural truth. True 

 to the speculative instinct of his country, Mayer drew large and weighty 

 conclusions from slender premises ; while the Englishman aimed above 

 all at the firm establishment of facts. The future historian of science 

 will not, I think, place these men in antagonism. To each belongs a 

 reputation which will not quickly fade for the share he has had, not 

 only in establishing the dynamical theory of heat, but also in leading 

 the way towards a right appreciation of the general energies of the 

 universe." 



Mr. Joule's paper " On the Calorific Effects of Magneto-Electricity, 

 and on the Mechanical Value of Heat," appears in extenso in " The 

 Philosophical Magazine " for 1843, The first part of the paper related 

 to the heat generated in a magneto-electric circuit. An electro-magnet 

 contained in a tube filled with water was rotated between the poles of 

 a powerful magnet, and the increase of temperature of the water was 

 observed. It then became a question of interest to find whether a 

 constant ratio existed between the heat developed and the mechanical 

 force used in turning the apparatus. The mechanical force was found 

 by winding twine round the spindle carrying the electro-magnet, passing 

 the twine over an easily-moving pulley, and attaching a scale pan. 

 The difference between the weights required to produce equal velocities 

 with the induced circuit complete and interrupted, gave the work done 

 against the electric forces which reappeared as heat. Under the cir- 

 cumstances of these experiments there were many attendant interfering 

 effects to be considered, and it was therefore almost inevitable that the 

 results should be somewhat discordant among themselves. The mean 

 result of the thirteen experiments recorded in the paper is thus stated : 

 " The quantity of heat capable of increasing the temperature of a pound 

 of water by one degree of Fahrenheit's scale is equal to, and may be 

 converted into, a mechanical force capable of raising 838 Ibs. to the 

 perpendicular height of one foot." A very much simpler apparatus, 

 consisting of a piston perforated by a number of small holes, working 

 in a cylindrical glass jar, is mentioned in a postscript. The mechanical 

 force required to urge the piston through the water was such, that one 

 degree per Ib. of water corresponded with a force capable of raising 

 770 Ibs. to a height of one foot. "I shall lose no time/' adds Mr. Joule, 

 "in repeating and extending these experiments, being satisfied that 

 the grand agents of nature are, by the Creator's fiat, indestructible; and 

 that wherever mechanical force is expended, an exact equivalent of 

 heat is always obtained." He applies this last view to the case of 

 chemical combination by suggesting that heat there results from the 

 collision of atoms rushing together. In their separated positions the 



