209

Chap. 5.]

Bag Pump.

No. 84. Bag Pump.

folds. The upper end of the bag should be firmly tied with a cord in a grooveturned out of the rim of the board at F. Intothis board is fixed the fork of the piston rod,and the bag is kept distended by a number ofwooden hoops or rings of wire, fixed to itat a few inches distance from one another, andkept at the same distance by three or fourcords binding them together, and Stretchingfrom the top to the bottom of the bag. Nowlet this trunk be immersed in the water : itis evident that if the bag be stretched fromthe compressed form which its own weightwill give it by drawing up the piston rod, itscapacity will be enlarged, the valve F willbe shut by its own weight, the air in the bagwill be rarefied, and the atmosphere willpress the water into the bag. When the rodis thrust down again, the water will come outat the valve F, and fill part of the trunk. Arepetition of the Operation will have a similareffect; the trunk will be filled, and the water will at last be dischargedat the spout.” The Operation is precisely the same as in No. 81.

“Here is a pump without friction and perfectly tight; for the leatberbetween the folds of canvas renders the bag impervious both to air andwater. We know from experiment that a bag of six inches diametermade of sail eloth No. 3, with a sheepskin between, will bear a columnof fifteen feet of water, and stand six hours work per day for a month,without failure; and that the pump is considerably superior in. effect to acommon pump of the same dimensions. We must only observe that thelength of the bag must be three times the intended length of the stroke,so that when the piston rod is in its highest position, the angles or ridgesof the bag may be pretty acute. If the bag be more stretched than this,the force which must be exerted by the laborer becomes much greaterthan the weight of the column of water which he is raising.”

But after all that can be said in favor of bellows pumps, they have theirdisadvantages. A prominent one is this: when the leather or other ma-terial of which they are formed is worn out, a practica! workman, whois not to be obtained in every place, is required to renew it. Unlike re-placing the leather on an ordinary ‘ sucker’, which a farmer or a sailor onship-board can easily accomplish, the Operation requires practice to per-form it efficiently, and the expense both of time and materials is muchgreater than that of similar repairs to the common pump. For these andother reasons, bellows pumps have never secured a permanent placeamong staple machines lor raising water, and the old cylindrical pumpstill retains the preeminence, notwithstanding the almost innumerable pro-jects that have been brought forward to supersede it.

The preceding machines resemble in some degree the apparatus fordrinking which the Creator has furnished to us and to such quadrupedsas do not lap. When an ox or a horse plunges his mouth into a stream,be dilates his ehest and the atmosphere forces the liquid up into his sto- <mach precisely as up the pipe of a pump*. It is indeed in imitation ofthese natural pumps that water is raised in artificial ones. The thorax isthe pump; the muscular energy of the animal, the power that works it;the throat is the pipe, the lower orifice of which is the mouth, and whichhe must necessarily insert into the liquid he thus pumps into his stomach ;

27