HYDRAULICS. 199 



duced transversely, upon being pulled back be- 

 comes immediately tight. 



These parts being provided, all that is necessary 

 to make the pump act as a fire-engine, after having 

 been used as a sucking-pump, is to plug up the 

 spout with the stopple. 



No particular mode being essential in the work- 

 ing of this pump, it may, according to choice, or 

 circumstances, be wrought by all the methods 

 practised with the common pump. In many cases, 

 however, it may be advantageous to have two of 

 them so connected, as to have an alternate motion, 

 in which case, one air-vessel, and even one suction- 

 piece, might serve both. 



Its principles admit of various modifications; 

 but as what is already mentioned, may be suf- 

 ficient to indicate its superiority over the common 

 and chain-pumps, and the advantages likely to 

 result from its general use, a further detail is 

 unnecessary. 



To this we may add, that the testimonies of 

 several navigators confirm in the fullest manner, 

 the hopes that were conceived of its utility, and 

 warrant the recommendation of it, as the best 

 adapted for the purpose of any pump hitherto 

 invented. 



The great desideratum in a piston is, that it be 

 as tight as possible, and have as little friction as is 

 consistent with this indispensable quality. 



The common form, when carefully executed, 

 has these properties in an eminent degree, and 

 accordingly keeps its ground amidst all the im- 

 provements which ingenious artists have made. It 

 consists of a hollow cylinder, having a piece of 

 strong leather fastened round it, to make it fit 

 exactly the bore of the barrel, and a valve or flap 



o 4 



