140 FERGUSON'S LECTURES. 



LECT ter therein, against the horses will be according to the 

 .^v^-x^ height to which it is raised, and the diameter of that 

 part of the pump in which the piston works, as we have 

 already observed. So that, by the same pump, an equal 

 quantity of water may be raised in (and consequently 

 made to run from) a pipe of a foot diameter, with the same 

 ease as in a pipe of five or six inches : or rather with more 

 ease, because its velocity in a large pipe will be less 

 than in a small one ; and therefore its friction against 

 the sides of the pipe will be less also. 47 



Note 47. Our attention may now be directed to some of the differ- 

 ent forms which may be given to the pistons and valves of a 

 pump. 



The great desideratum in a piston is, that while it be as tight 

 as possible, it should have as little friction as is consistent with this 

 indisper.sable quality. The common form, when carefully executed, 

 possesses these properties in an eminent degree. This piston is a sort 

 of truncated cone, generally made of wood not apt to split, such as 

 helm or beech. The small end of it is cut off at the sides, so as to 

 form a sort of arch, by which it is fastened to the iron rod or spear. 

 The two ends of the conical part may be hooped with brass. This 

 cone has its larger end surrounded with u ring or band of strong 

 leather fastened with nails, or by a copper hoop, which is driven 

 on it at the smaller end. The further this band reaches beyond the 

 base of the cone, the better ; and the whole must be of uniform thick- 

 ness all round, so as to suffer equal compression between the cone 

 and working barrel. The seam or joint of the two ends of this band 

 must be made very close ; but not sewed or stiched together, as that 

 would occasion bumps or inequalities, which would spoil its tightness ; 

 and no harm can result from the want of it, because the two edges will 

 be squeezed close together by the compression in the barrel. Nor is 

 it by any means necessary that this compression be great : this is a 

 very detrimental error of the pump-makers. It occasions enormous 

 friction, and destroys the very purpose which they have in view, viz. 

 rendering the piston air-tight; for it causes the leather to wear through 

 very soon at the edge of the cone, and it also wears the working bar- 

 rel. This very soon becomes wide in that part which is continually 

 passed over by the piston, while the mouth remains of its original 

 diameter, and it becomes impossible to thrust in a piston which shall 

 completely fill the worn part. Now a very moderate pressure is 

 sufficient for rendering the pump perfectly tight, and a piece of glove- 

 leather would be sufficient for this purpose, if loose or detached from 



