1172
THEORY AND PRACTICE OF ENGINEERING.
Book IT.
To avoid the friction of the piston against the inner surface of the pump-barrel, whichwould be considerable if it operated on its whole length, its diameter is a little less thanthat of the pump-barrel, so as to leave a small interval between them : to prevent thecommunication with the external air, which would be an obstacle to suction, and that tlx?water when forced may not issue through the mouth of the barrel CD(fig. 1778.), a very simpleand ingenious contrivance is used. The entrance L L of the barrel is provided with a flangeKL, which surrounds and is cast with it; on this are applied two or three roundles ofleather E F G, bent round the inner surface of the barrel, and then a copper ring whoselesser diameter is a mean between that of the pump-barrel and that of the piston. Uponthis other roundles are placed, A B Z, bent like the preceding, but in an opposite direction,the whole covered by a second copper ring, H II , whose lesser diameter, 11, is equal to thatof the barrel. This ring is united to the flange K L by screws, CD, fitted with nuts; thusthe middle ring serves as a guide to the piston, which only touches the leather Z G, withwhich it is intimately united, for as there is always water in the cistern XY (fig. 1777), theleather is continually swelled. This water, not being able to escape, prevents the external airfrom entering the pump-barrel, and when necessary the leathers may be removed, and thepump repaired without taking it to pieces. In order that the water from the pump may keepthe cistern always full, a small cock It is added, which communicates with the barrel andis closed by a key S : when the piston plunges, the water mounts in the cock, and byturning the key S is admitted into the cistern ; as the violence with which it is pressed bythe piston would make it rush out impetuously, a plate of copper, Z, is opposed to it, sup-ported on four branches. This cock also serves at the first moment to expel the air fromthe barrel quicker than if it were driven out through the branch, it being opened and shutalternately at each stroke of the piston. In all these pumps, the water only ascends thebranch at intervals, that is to say, when the piston plunges, consequently the time of suctionis lost. Large pumping machines, as that of the Samaritainc .
at Paris, therefore have always at least two separate barrels. A, B, S',
fig. 1779., uniting in the same ^ Hjibranch C by the pipes D and E, 1
so that when the piston F sucks,the other G forces, and the watercontinually ascends. In thismanner, the pumps of the Sa-maritaine, at Paris, were con-structed.
Fig. 1780. shows a pumphaving a pipe CAB divided intotwo equal parts A B and A C ,forming two barrels uniting inthe same branch QI) It, withwhich they communicate by twoholes G and H. At this end thebranch is elliptic, as well as thetwo holes G and H, which openand shut alternately by means ofa valve common to both ; belowis shown the valve in face, andsideways, with a section throughthe middle; this valve is entirelyof copper, either solid or hollow,provided it be strong, and itshows that it works on a hinge atE, between the holes G and H,which is its centre of motion.
The frame Z carries twopistons, working in opposite di-rections ; for if we suppose themachine plunged in water to tbe seen that when the frameof the piston M opens, and thebarrel A B ; that in the second,the piston X passes through the hole H into the ascending branch, supports thevalve F in the situation represented, and while it slides along the face E K, theother EI leans against the orifice G, which it keeps shut. But as soon as the framedescends, the valve N closes, the other L opens, and that in the middle changesposition ; the water in the pump-barrel A B passes through the hole G, to l>e forced in itsturn up the ascending branch, and the hole H is closed by the face E K. On the other
Fig. 1779. pcmt at the
SAMAKITAINE.
Fig. 1780.
G F>K
LiPe