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OUR PHYSICAL WORLD 



The general method of operation of the modern steam engine 

 is very much the same as that of Watt's fire engine, though very 

 many improvements in details have been made in it. The 

 boiler is commonly what is known as the tubular type in which 

 the draft carries the heat from the fire box up between numer- 

 ous pipes or tubes containing the water that is to be turned to 

 steam. These tubes present a much larger heating surface than 

 the old type of kettle-like boiler, and steam can be made much 

 more rapidly. The modern boiler is so well made that it stands 

 high pressures, and the steam is sent to the cylinders with a 

 pressure of several hundred pounds to the square inch. 



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FIG. 72. Diagram of a modem steam engine 



When the engineer opens the throttle of the engine it lets 

 steam from the boiler into the steam chest that lies next to the 

 cylinder. Sliding valves between steam chest and cylinder let 

 steam first into one end and then into the other, at the same 

 time others open to let out the exhaust steam. These valves 

 are operated by a rod attached to the eccentric or similar device. 

 The method of operation of this portion of the engine should be 

 plain from the study of the accompanying diagram (Fig. 72). 

 The exhaust steam from the high-pressure cylinder may be dis- 

 charged directly into the air through the smokestack or it may 

 go to a condensing chamber in the so-called condensing engine 

 or it may enter another steam chest and cylinder that works at 

 less pressure before going to the condenser, for the work power 

 of the steam in high-pressure engines is not taken out of it entirely 



