VjO HYDRAULICS, 



exert a much greater force in opposition to a pull 

 than to compression. The collar of leather round 

 the piston-rod, is found by experience to be very 

 impervious to water; and though it needs but 

 little repair, yet the whole is very accessible, and 

 in this respect much preferable to the common 

 pump in deep mines, where every fault of the pis- 

 ton obliges us to draw up some hundred feet of 

 piston-rods. By this addition, too, any common 

 pump for the service of a house may be converted 

 into an engine for extinguishing fire, or may be 

 made to convey the water to every part of the 

 house; and this without hurting or obstructing its 

 common uses. All that is necessary is to have a 

 large cock on the upper part of the working- 

 barrel, opposite to the lateral pipe in this figure. 

 This cock serves for a spout, when the pump is 

 used for common purposes; and the merely shutting 

 this cock converts the whole into an engine for 

 extinguishing fire, or for supplying distant places 

 with water. It is scarcely necessary to add, that, 

 for these services, it will be requisite to connect 

 an air-vessel with some convenient part of the 

 rising-pipe, in order that the current of water may 

 be continual. 



It is of considerable importance, that as equable 

 a motion as possible be produced in the main pipe, 

 which diminishes those strains which it is otherwise 

 liable to. The application of an air-vessel at the 

 beginning of the pipe answers this purpose. In 

 great works, it is usual to effect this by the alternate 

 action of two pumps. It will be rendered still 

 more uniform, if four pumps be employed, suc- 

 ceeding each other at the interval of one quarter 

 of the time of a complete stroke. 



But ingenious men have attempted the same 

 17 



