1890.] on The Scientific Work of Joule. 3 



apparatus by the suiToimding air, Davy made an experiment in vacuo, 

 isolating the apparatus by means of ice ; and found that under such 

 conditions sufficient heat could be produced to melt wax placed in the 

 receiver. The lecturer here showed an exj^eriment illustrating the 

 production of water by the friction of two pieces of ice in vacuo, 

 under conditions of temperature much more severe than those of 

 Davy's experiment. 



Following Davy, Young devoted a great deal of attention to the 

 subject, and by 1812 he and Davy had quite changed their opinions, 

 and had adopted the view that heat and motion were convertible 

 effects. 



Having by July 1843 assured himself of the principle of his dis- 

 covery, Joule now devoted himself to the elaboration of methods 

 of working, modifying and repeating experiments in various ways, 

 but always approaching nearer and nearer to exactness, as shown by 

 the following Table of results : — 



Joule's values of the Mechanical Equivalent op Heat. 



Kilogramme 

 metres. 



Magneto-electric currents 1843 460 



Friction of water in tubes „ 424 * 6 



Diminution of heat produced in a battery current when the 



current produces work „ 499 



Compression of air 1845 443*8 



Expansion of air „ 437 • 8 



Friction of water „ 488*3 



„ „ „ 1847 428-9 



„ „ „ 1850 423-9 



,, „ mercury „ 424*7 



„ „ iron ■. „ 425*2 



Heat developed in Daniel's cell „ 419 - 5 



„ „ in wire of known absolute resistance .. .. 1867 429*5 



Friction of water in calorimeter 1878 423-9 



Prof. Dewar here exhibited and explained the action of the 

 original calorimeter used by Joule. It was seen to consist of a set of 

 vanes which were made to revolve in water by the falling of known 

 weights through a definite and known height, the heat produced being 

 due (after making the necessary deduction for the friction due to the 

 momentum of the weights) entirely to the friction of the fluid. It 

 was found that whatever fluid was employed, the same definite results 

 were obtained : — a production of heat in the liquid bearing a constant 

 relation to the unit of mechanical energy expended. The extreme 

 delicacy of Joule's apparatus, and the marvellous accuracy of his 

 observations were shown by the fact that working with weights of 

 29 lbs. each, and repeating each observation 20 times, the total increase 

 of temperature did not exceed half a degree Fahrenheit. In contrast 

 to this the lecturer showed, by means of apparatus kindly lent by 

 Prof. Ayrton, the method now employed for repeating Joule's work 

 and arriving at substantially the same results b}' much simpler means 



B 2 



