THE STEAM-ENGINE. 363 



the limits due to closing the throttle-valve and throwing 

 it wide open. Further than this, smoothness of motion is 

 secured by attaching a heavy fly-wheel to the shaft of the 

 engine. A little reflection will show that the fly-wheel 

 also acts as an accumulator of energy. 



575. The Safety- Valve. The safety-valve is a 

 necessary part of every steam-boiler. It consists of a 

 valve, F, held down over an opening in the top of the 

 boiler by means of a spring or a 



loaded lever of the second class. 



The force with which the valve 



is held down is to be less than 



the strength of the boiler, i. e., 



the force must be such that the FIG. 270. 



valve will open before the tension 



of the steam becomes dangerous. On steamboats, the 



weight, W, is generally locked in position by a Government 



officer. 



576. Non- Condensing 1 Engines. When the 

 steam is forced out at N (Fig. 267), it has to overcome an 

 atmospheric pressure of 15 pounds to the square inch. 

 This must be deducted from the total tension of the steam 

 to find the available power of the engine. Such an engine 

 is known as a non-condensing engine. It may be recog- 

 nized by the escape of steam in puffs. It is generally a 

 high-pressure engine. The railway locomotive is a high- 

 pressure, non-condensing engine. 



577. Condensing Engines. The steam may be 

 conducted from the exhaust pipe N (Fig. 267) to a chamber 

 called a condenser. Steam from the cylinder and a jet of 

 cold water being admitted at the same time, a vacuum is 



