LESSONS IN PHYSICS. 69 



While experimenting in the line of conservation of energy, Mr. 

 Joule of England discovered that a quantity of heat necessary 

 to raise one pound of water one degree Far. would raise 772 Ibs. 

 one foot high, or would do 772 foot-pounds of work, which is 

 called the mechanical equivalent of heat. A steam boiler and 

 engine constitutes a machine whose purpose is to transform heat 

 into mechanical motion or work. The essential parts are the 

 furnace, the boiler, the steam pipe, the steam chest and slide 

 valves, the cylinder, the piston, the exhaust pipe, piston rod, 

 crank and shaft. The fire in the furnace expands the water in 

 the boiler into steam. The steam pipe conveys this to the steam 

 chest from which it passes through the slide valves into the cylin- 

 der, where expanding now against one, then against the other end 

 of the piston, gives a forward and backward movement, which is 

 conveyed by the piston rod to the crank, where the backward and 

 forward movement is changed into a rotary movement. The 

 steam after expansion in the cylinder passes out through the 

 exhaust pipe. The shaft carries a heavy balance wheel which 

 serves as a reservoir of energy, which is needed to make the rota- 

 tion of the shaft and all machinery connected with it uniform, so 

 that the sudden changes connected with the driving power or 

 resistance are avoided by means of the inertia of this wheel. 

 Motion may be communicated from the shaft to any machinery 

 desirable. While this machine is for the purpose of transforming 

 heat into mechanical work, yet with all modern improvements 

 not more than 20 per cent, of the heat units can be utilized as 

 work. 



