458 RECAPITULATION. 



17. In pump-work, all contractions, or sudden 

 enlargements, in the pipes, should be avoided. 



18. The steam engine was originally invented 

 by the Marquis of Worcester, but was first put in 

 practice to any extent, by Captain Savary. 



19. Savary's engine had no lever, but acted by 

 the immediate pressure of the steam upon the 

 water. 



20. Newcomen improved it, by adding a lever, 

 or beam, and attaching to it a piston which worked 

 in a cylinder. Upon this piston the pressure of the 

 atmosphere is made to act, by forming a vacuum 

 underneath it. 



21. Mr. Watt improved the cylinder, by sur- 

 rounding it with bad conductors of heat; and this 

 prevented a waste of steam, by cooling. — He also 

 condensed the steam, to form the vacuum under 

 the piston, in a separate vessel. Instead of de- 

 pressing the piston by the pressure of the atmo- 

 sphere, he used the force of steam introduced 

 above it, while the piston was raised up again by 

 the load at the other end of the beam. His last 

 improvement is the double steam-engine ; in which 

 the piston is forced both up and down, by the im- 

 mediate pressure of the steam. 



ABSTRACT OF ACOUSTICS. 



1. The science which treats of sound is called 

 Acoustics. 



2. Sound is occasioned by the vibrations of the air 

 striking upon the tympanum, or drum of the ear 



3. Without air there would be no sound. 



4. Eehos are owing to the reflection of the 

 waves or vibrations of the air by solid bodies. 



