358 THERMODYNAMICS. 



experiments were equal in number and variety to the im- 

 portance of the subject. He showed that the mechanical 

 value of a heat unit is 772 foot-pounds, referring to 

 the Fahrenheit degree; 1390 foot-pounds refer- 

 ring to the centigrade degree. This is expressed by 

 saying that the "mechanical equivalent" of heat is 772 or 

 1390 foot-pounds. ( 514 [>].) 



(a.) A change in the unit of weight will not affect these numbers, 

 which must not le forgotten. If the heat unit be " kilogram-Fahren- 

 heit," the equivalent will be 772 foot-kilograms ; if the thermal unit 

 be " gram-centigrade," the equivalent will be 1390 foot-grams. A 

 change in the unit of length will work a change in the number 

 representing the equivalent. If the equivalent for a "kilogram- 

 centigrade " heat unit be desired in kilogrammeters instead of foot- 

 kilograms, the number 1390 must be divided by the ratio between 

 the values of a foot and a meter, becoming thus 424. kilogrammeters. 



567. The Use of Joule's Equivalent. The 



use of the mechanical equivalent of heat may be well shown 

 by the solution of a problem. 



(a.) If a cannon-ball weighing 192.96 pounds and moving with a 

 velocity of 2000 feet per second, be suddenly stopped and all of its 

 kinetic energy converted into heat, to what temperature would it 

 warm 100 pounds of ice-cold water ? 



MB* 192.96x4000000 ^ OAAAAAA - 

 Kinetic energy = - = - --- = 12000 ? 00 foot-pounds. 



12000000 + 772 = 15544 + heat units. 



15544 _j_ 100 155.44 heat units for each pound of water. This 

 would raise the temperature 155.44 F., leaving it at 187.44 F. Ans. 



(&.) Knowing the weight of the earth and its orbital velocity, we 

 may easily compute the amount of heat that would be developed by 

 the impact of the earth against a target strong enough to stop its 

 motion. The heat thus generated from the kinetic energy of the 

 earth would be sufficient to fuse if not vaporize it, equalling 

 that derivable from the combustion of fourteen globes of coal 

 each equal to the earth in size. After the stoppage of its orbital 

 motion it would surely be drawn to the sun with continually 

 increasing velocity. The heat instantaneously developed from 



