RECAPITULATION. 457 



4. A syphon is a bended pipe of unequal legs. 

 The cause of its action in emptying vessels is 

 owing to the pressure of the atmosphere added to 

 the preponderance of weight in the longest leg. 



5. Pumps for raising water are of three kinds : 

 the sucking, forcing, and lifting-pump. 



6. The water in a sucking-pump is raised from 

 the well by the pressure of the atmosphere ; and it 

 can be raised by this means only S3 feet. 



7. A lifting-pump, not depending upon this, 

 may raise the water to any height, according to 

 the power applied. 



8. A forcing-pump is also unlimited, in regard 

 to the height to which it may raise water. 



9. An air-vessel is added to a forcing-pump, to 

 give a more equable stream. 



10. A constant stream is also produced by two 

 barrels, with pistons moving up and down alter- 

 nately. 



11. The chain-pump also produces the same 

 effect, and has very little friction. 



12. Buchanan's pump is superior to the chain- 

 pump, and is one of the best yet invented. 



13. There are many contrivances to avoid fric- 

 tion in pumps ; but in great works, the friction of 

 the piston is of little importance. 



14. Plungers are pistons that nearly fill the 

 working-barrel: these do not act upon the prin- 

 ciple of the pressure of the atmosphere. 



15. Valves in pumps are of various constructions: 

 the most usual and best are the clack-valve, the 

 button and tail-valve, the conical-valve, and the 

 globular -valve. 



16. It is immaterial whether a pump be placed 

 perpendicular to the well or not, provided it have 

 a communication with it by pipes. 



