Chap. XVII.
] 181
in ascending and descending to work perpendicularly: it being impossible to make the roddo this at all times, especially when worked by a crank, great freedom of action must beallowed in forcing-pumps ; therefore it is better to hook it to the piston than to attach it.
Although the preceding piston is one of the best, it must be acknowledged that after acertain time, when the leather has successively swelled to compensate for the decreasecaused by friction, the adhesion will not be sufficient to prevent the passage of water whenforced, if the column be very high, for the resistance caused by its weight will always bethe same, while the adhesion of the piston will continue to diminish. To remedy thisthere must be a power proportioning its adhesion to the pressure exerted in forcing : thishas been effected by a copper cylinder gk, hollow and piercedwith several holes covered above by a plate ah of the samematerial, both cast together with the flange IK, serving toattach the cylinder to a second plate cd, similar to the first, withtliis difference, that it has a hole of a diameter equal to theinterior of the cylinder, to which a cup valve is fitted, in such amanner that the tail is retained between the flange 1 K and theplate, the whole being secured with nuts and screws : on theedge of each plate is a groove to receive the hem of a leatherpurse of a cylindrical shape, for which the plates serve asends. Pitched twine is used to unite them, well tightenedround the leather, forming a drum, and having only an open-ing at the bottom when the valve at K is raised; the pistonis attached to the rod II, having three or four branches I G,united to the plate ah by nuts and screws. AVater beingpoured into the pump-barrel until it is three-fourths full, thepiston is introduced, which will enter without difficulty ; butwhen it descends lower the air contained beneath, being com-pressed, will raise the valve, pass into the cylinder <//t, thenceinto the drum, and, continuing to descend, the water will passalso, until the piston arrives at the hole N O; the air and thewater then swelling the drum, the leather will begin to adhereto the pump-barrel, feebly it is true, but sufficiently to preventthe introduction of the external air when the piston is raised,the valve instantly closing.
As the piston works and expels the air from the suction-pipe,the water will rise in it, and enter the barrel: when arrivedthere, the piston forcing it will itself receive a part, which will a {£==. »compress the air at each stroke into a still less volume, and the s^=action of the piston becoming stronger as the water is raised toa greater height in the branch, the air in the drum will acquirestill more elasticity, and press the leather more and more againstthe barrel, the water therein acting in the same manner as theair. When the branch is full, the elasticity of the air will be inequilibrio with the column of*water, of whatever height it maybe, and whether the piston forces or sucks its adhesion willalways be the same : when the barrel is not sufficiently cylindrical, this defect, whichwould be very great in any other case, will be of no account in this, since the surface ofthe piston being flexible will adapt itself to whatever opposes it.
The piston just described cannot lose water, its surface being perfectly applied to thebarrel; but as from this adhesion great friction results, the leather will not last long: it istherefore necessary, in order to avoid its constant renewal, to place several, one over theother, to strengthen the purse, which will not be the less flexible, but adapt itself to thebarrel; the friction in this case being very different from that occasioned by the contact oftwo hard bodies, it is highly desirable to get rid of friction entirely, but in doing so, caremust be taken that other inconveniences are not produced, which may overbalance the oneadvantage.
Messrs. Gosset and De la Deuille, while engaged on a very ingenious hydraulic machine,invented a piston entirely free from friction, which may be used independently of themachine : the piston may be of any size, even 3 feet in diameter ; that described is 15 inches :the barrel consists of two wooden plates 28 inches in diameter, and 5 thick ; in the middleof each is a cylindrical hole 15 inches in diameter and 2^ deep, forming two boxes placedagainst each other ; their section is represented by the rectangles A B C D and E F H G.The piston consists of a circular plate, Y L, 1 inch thick, the diameter of which ought to berather less than the void TO Q.V, to facilitate the play: this plate is applied to a largecircle of leather, or to several, if one is not sufficiently strong, so as to project 6 or 7 inchesall round the edge; the plate is placed in the bottom of the box ST VX, and the excess isfolded all round the edge E S X G ; the other box, A B C D, is then applied to the preceding,