528 HISTORY OF SCIENCE. 



therefore cause the rotation of the spindle. The height through which 

 the weights descend is measured by the rod, r r 1 , and the operation 

 is several times repeated, the spindle s being only turned by the de- 

 scending weights. A very delicate thermometer, /i, indicates the tem- 

 perature of the liquid. The value found by experiments in 1847 was 

 781-8 foot-lbs. Several years' experience in experiments of this kind 

 enabled Mr. Joule to apply to a series, completed in 1849, all possible 

 refinements and corrections, and the mean results of more than 100 

 determinations were 



772*69 from the friction of water, 

 774-08 mercury, 



774-98 cast iron. 



He finally fixed upon 772 foot-lbs. as the mechanical equivalent of 

 the heat which will raise the temperature of a pound of water one 

 degree Fahrenheit between 55 and 60 F. 



Quite recently (1879) Professor Rowland, of the newly-founded 

 John Hopkins University at Baltimore, U.S., has conducted a series 

 of experiments to re-determine the mechanical equivalent by an ar- 

 rangement invented by himself. The calorimeter with its fixed and 

 revolving paddles, are like those in Joule's apparatus (Fig. 244) ; but 

 the work done on the water is determined in this way: the spindle 

 carrying the revolving paddles is turned by power derived from a 

 steam-engine, and the number of revolutions is automatically indicated. 

 The tendency of the vessel itself to rotate is, in Rowland's apparatus, 

 opposed by cords that support weights, and these therefore furnish 

 the measure of the mechanical force applied to the water. The water 

 in this apparatus may be raised to the boiling-point by the mere 

 churning action of the paddles. The numerical results are nearly 

 identical with those of Joule ; for the mechanical value of a pound of 

 water raised from 80 to 81 F. was found by the Baltimore experi- 

 ments to be 7757 foot-lbs. 



The principle of a mechanical equivalent of heat, suggested by Grove 

 and Mayer, and firmly established on an experimental basis by Joule, 

 is far-reaching and of vast importance. It gave to science not merely 

 a new conception, but a law which, worked out in its consequences 

 and relations, was found to include in its highest generalization the 

 widest laws of all the several branches of science. The result has been 

 that within the last thirty years each branch of science has been found 

 capable of presenting a new aspect in its relation to other branches. 

 They are now connected and controlled by the great laws of energetics, 

 which assert the constant inter-equivalence of different forms of force, 

 and by the law of the Conservation of Energy. The scientific meaning 

 of the term energy should be clearly understood. Energy may be 

 defined as the power of doing work. Work is another term of which 



