SOURCES OF HEAT. 263 



the product obtained by the multiplication of the weight by 

 the square of its velocity. The first method, however, be- 

 cause it is the more convenient, is the one usually adopted ; 

 and the numbers obtained therefrom may easily be expressed 

 in other units. 



The product resulting from the multiplication of the num- 

 ber of units of weight and measures of height, or, as it is 

 called, the product of mass and height, as well as the pro- 

 duct of the mass and the square of its velocity, are called " vis 

 viva of motion," " mechanical effect," dynamical effect," 

 " work done," " quantite de travail" &c. &c. 



The amount of mechanical work necessary for the heating 

 of 1 kilogramme of water 1 C has been determined by ex- 

 periment to be = 367 Km ; therefore Km = 0-00273 units 

 of heat.* 



A mass which has fallen through a height of 367 metres 

 possesses a velocity of 84-8 metres in one second ; a mass, 

 therefore, moving with this velocity originates 1 C. of heat 

 when its motion is lost by percussion, friction, &c. If the 

 velocity be two or three times as great, 4 or 9 of heat will 

 be developed. Generally speaking, when the velocity is c 

 metres, the corresponding development of heat will be ex- 

 pressed by the formula 



0-000139 Xc 2 . 



* This essay was published in 1845. At that time de la Roche and 

 Berard's determination of the specific heat of air was generally accepted. 

 If the physical constants used by Mayer be corrected according to the re- 

 sults of more recent investigation, the mechanical equivalent of heat is 

 found to be 771*4 foot-pounds. Mr. Joule finds it = 772 foot-pounds. 

 TH. 



