THE STEAM ENGINE 335 



by its own slings and brackets. Then it will usually be 

 found that several lengths have to be altered before the 

 flange faces come into alignment. Not until this has been 

 done and every pair of flanges inspected by some responsible 

 person, should the various lengths be bolted together per- 

 manently. 



When a number of small or moderate-sized engines are 

 connected with the same pipe system and stand on the same 

 foundation, or in the same building, it is sometimes difficult 

 to prevent the pipes from vibrating and at the same time 

 insure the necessary freedom for expansion and contraction. 

 Installations of this kind should therefore be arranged in 

 such a way that the pipes are quite free to move in one direc- 

 tion, parallel with their length, while movement in other 

 directions should be restricted so far as possible. 



High-speed engines are those whose fly-wheels rotate at a 

 high speed; i.e., make a large number of revolutions per 

 minute. Such engines are less expensive to operate than low- 

 speed engines, because the power of an engine depends upon 

 area of its piston, the mean pressure of steam, and the speed 

 at which its fly-wheel rotates. Therefore by doubling the 

 speed, an engine may be built very much smaller and cheaper 

 per horse-power. Engines of this type are used for driving 

 electrical machinery, which requires high speed of rotation 

 and uniform angular velocity. 



376. Horse-Power. The power of a steam engine is com- 

 monly designated as horse-power. One horse-power is a force 

 strong enough to raise 38,000 Ibs. one foot high in one minute; this 

 has been found to be about what a very strong horse could do 

 working 8 hrs. a day. An engine of 100 H. P. would be, of 

 course, able to do a hundred times as much as this. A steam- 



