THE STEAM ENGINE 



329 



increase boiler pressures, so also the working parts and the 

 structure of steam engines have been improved and strength- 

 ened, until now the weight of engine per horse-power of capac- 

 ity and the cost of the plant are much less than in Watt's time. 



368. Principal Parts of Steam Engines. The principal 

 parts of a simple engine (Fig. 169) are the frame, cylinder, 



FIG. 169. Steam Engine. 



A Cylinder D Steam Inlet G Crank 



B Piston E Steam Ports //Slide Valve Eccen- 



C Slide Valve /^Exhaust Port trie 



piston, rods, eccentric, crank shaft, governor, and wheels. 

 The cylinder is the long, round, iron barrel or tube in which 

 the piston works. The piston is a disk, fitting into the cylin- 

 der and dividing it into two compartments. Packing rings 

 are provided to make it steam-tight. The piston moves back 

 and forth, forced by the steam which is alternately admitted 

 on each side of it by means of openings called ports. That 

 is, steam is allowed to enter the cylinder by one port, and 

 forces the piston along, the other port being opened by the 

 slide valve into the exhaust port during this stroke. As soon 

 as the piston has reached the end of the cylinder, the first 

 port closes for the admission of steam, while the second port 



