PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 



249 



tliis important subject was resumed, and wlio again recalled it to the attention 

 of pliysicists in a treatise published, in 1842, in the Annals of Chemistry and Phar- 

 macy, under the title of Obsrnrifiojis on the forces of inanimate 'nature, was Mayer, 

 a prat:tising physician of Ileilbronn, 



III. THE MECHANICAL EQUIVALENT OF HEAT. 



Mayer first enunciated the idea that, as a definite proportion universally exists 

 between cause and effect, there must always, in the production of heat by mechani- 

 cal means, be an invariable projiortion between the heat generated and the 

 mechanical power consumed for that purpose ; and, in fact, he thus earl}' established, 

 with closely apprt)ximate exactness, the mechanical ecpiivalent of heat. This 

 ■was, at a later period, still more accurately determined through the researches 

 of Joule and Hirn. 



In 1843 the observation was made by Joule (Phil. Mag., vol. xxiii) that, in 

 the passage of water through a naiTow tube, heat is generated, and that a 

 mechanical power of 770 foot-pounds* is consumed in raising the temperature 

 of one pound of water to 1° F., a result which, as we shall see, is not very differ- 

 rent from that obtained by compression of the air. 



Joule sought, also, to ascertain by other methods the proportion of the 

 heat generated by friction to the mechanical power thereby expended. In a 

 copper vessel (A, Fig. 3,) a paddle wheel, whose construction is represented 



Fig. 3. 



Fig. 4. 



in Fig 4, was so disposed as to be capable of revolving around a vertical 

 axis. Eight paddles of thin plate, at an angle of 45° with one another, are 

 placed at the height h, eight others at the height g. They move between metallic 

 plates, which are fixed to the wall of the vessel ; four of 

 these plates standing at right angles to one another, being 

 at the height/, and four others at the height c. 



The vessel A, which stands on a wooden pedestal, is 

 filled with water, and the revolution of tlie paddle-wheel 

 is effected, in the manner represented in the figure, by 

 means of the weights B and D, which by their descent 

 co-operate in communicating motion to the axis of the 

 wlieel, and have a fall of about G3 inclies. After these 

 weights had reached the floor, by withdrawing the peg s, 

 the connection of the cylinder V with the axis of rotation 

 of the paddle-wheel was severed, the weights B and D were again wouml up, 



* A foot-pound is the power^evcloped in tlie fall of a weight of one pound throuj^h a height 

 of one foot. An English unit of heat is the ipiantity required to raise a pound ot water one 

 degree Fahrenheit. A kilogram-metro is the power developed iti the tall of a kilogram 

 (2.204 pounds) through one metro. A French unit of heat is the quantity required to raise 

 one kilogram of water one degree centigrade. 



