OF HYDRAULIC ENGINES. 141 



And the force required to raise water depends not LECT 

 upon the length of the pipe,, but upon the perpendicujar y^T^ 

 height to which it is raised therein above the level of 



(he solid cone ; for suppose such a loose and flexible, but impervious, 

 band of leather put round the piston, and put into the barrel ; and let 

 it even be supposed that the cone does not compress it in the smallest 

 degree to its internal surface. Pour a little water carefully into the 

 inside of this sort of cup or dish ; it will cause it to swell out a little 

 and apply itself close to the barrel all round, and even adjust itself to 

 all its inequalities. Let us suppose it to touch the barrel in a ring 

 of an inch broad all round. We can easily compute the force with 

 which it is pressed. It is half the weight of a ring of water an inch 

 deep and an inch broad. This is a trifle, and the friction occasioned by 

 it not worth regarding ; yet this trifling pressure is sufficient to make 

 the passage perfectly impervious, even by the most enormous pressure 

 of a high column of incumbent water : for let this pressure be ever so 

 great, the pressure by which the leather adheres to the barrel always 

 exceeds it, because the incumbent fluid has no preponderating power 

 by which it can force its way between them, and it must insinuate 

 itself precisely so far, that its pressure on the inside of the leather 

 shall still exceed, and only exceed, the pressure by which it endea- 

 vours to insinuate itself : and thus the piston becomes perfectly tight 

 with the smallest possible friction. This reasoning is perhaps too refined 

 for the uninstructed artist, and probably will not persuade him. To 

 such we would recommend an examination of the pistons and valves 

 contrived and executed by that artist, whose skill far surpasses our 

 highest conceptions, the all-wise Creator of this world. The valves 

 which shut up the passages of the veins, and this in places where an 

 extravasation would be followed by instant death, are cups of thin 

 membrane, which adhere to the sides of the channel about half way 

 round, and are detached in the rest of their circumference. When the 

 blood comes in the opposite direction, it pushes the membrane aside, 

 and has a passage perfectly free. But a stagnation of motion allows 

 the tone of the (perhaps) muscular membrane, to restore it to its natural 

 shape, and the least motion in the opposite direction causes it instantly 

 to clap close to the sides of the veins, and then no pressure whatever 

 can force a passage. \V'e shall recur tp this again when describing 

 the various contrivances of valves, &c. t l 'hat we have said is enough 

 for supporting our directions for constructing a tight piston. But we 

 recommend thick and strong leatber. while our present reasoning 

 seems to render thin leather preferable. Tf the leather be thin, and 

 the solid piston in any part does not press it gently to the barrel, there 

 will be in this part an unbalanced pressure of the incumbent column of 

 water, which would instantly burst even a strong leather bag : but 



