Chap. 3.] Modes of Working Pipes. 461 



rower compafs, at each explofion, its gradual reftitu- 

 tion to its firft dimenfions is what regularly carries on 

 the dream between the ftrokes, and renders it continual 

 during the operation of the machine. 



Various agents are employed to put pumps in ac- 

 tion, fuch as men, hcrfes, ftreams of water, vapour, 

 wind, &c. Small pumps, fuch as thofe in common 

 wells, or fire engines, are generally wrought by men. 

 When a confiderable quantity of water is required to 

 be raifed, the moving power muft be increafed; and 

 in order that a regular effort may be employed, or 

 very nearly fo, feveral additions are occafionally made 

 to the mechanifm of pumps, fo that when one fet of 

 piftons defcend another fet may be made to rife by the 

 fame force. The great engine at Marli is wrought by 

 an apparatus of this kind. 



The action of thefe engines depends upon the regu- 

 larity of the alternate motion of the fuckers or valves. 

 It follows then, that thefe mould be fo conftructed and 

 difpofed as perfectly to retain the water when they are 

 clofed, and to open eafily when the action of the ma- 

 chine is directed to that object:, 



V. .Of the motion of water in conduit pipes. 



When it is required to conduct water from one 

 place to another, it is very clear that the conducting 

 pipes muft be longer than the fpace through which it 

 is to be conveyed. 



In treating of the difcharge of water through addi- 

 tional pipes, I have taken but a curfbry notice of the 

 refiftance by friction, becaufc there it is fcarcely per- 

 ceptible; but it is not the fame with refpect to pipes 

 of confiderable extent ; the friction of the water againft 

 their fides leflens confiderably the velocity of the flow- 

 ing 



