THERMOD YNAMICS. 



357 



(a.} Attach a brass tube 10 cm. long, about 2 cm. in diameter and 

 closed at the bottom, to a whirling table. Partly fill the tube with 

 cold water and cork the open end. Press the tube between two 

 pieces of board hinged together as shown in the figure. The boards 



FIG. 265. 



should have two grooves for the reception of the tube ; the inner 

 faces of the boards may be covered with leather. When the machine 

 is set in motion the friction warms and soon boils the water. The 

 steam drives out the cork with explosive violence. 



565. First Law of Thermodynamics. When 

 heat is transformed into mechanical energy or 

 mechanical energy into heat, the quantity of heat 

 equals the quantity of mechanical energy. This 

 principle is the corner-stone of thermodynamics. It is 

 a particular case under the more general law of the Con- 

 servation of Energy. 



566. Joule's Equivalent. It is a matter of great 

 importance to determine the numerical relation between 

 heat and mechanical energy ; to find the equivalent of a 

 heat unit in units of work. This equivalent was first 

 ascertained by Dr. Joule, of Manchester, England. His 



