OF
AN AIR PUMP,
Or Essay towards completing the Vacuum.
The rapidity with which a vacuum is formed by an Air Pump, depends on the ratio between the contents of the receiver and those of the pump barrels. If the latter be just equal to the contents of the former, (which is a very large proportion) the exhaustion will follow this series:—there will remain in the receiver after each stroke, the first contents being 1, 1⁄2, 1⁄4, 1⁄8, 1⁄16, 1⁄32, 1⁄64, 1⁄128, 1⁄256, &c. But if the pump barrel contains twice the volume of the receiver—then the remaining air, after the strokes, will be 1⁄3, 1⁄9, 1⁄27, 1⁄81, 1⁄243, 1⁄729, 1⁄2187, 1⁄6561, &c. being much nearer to a vacuum than on the former supposition.
To meet this case, then, I have thought a water pump might be used: that is, a barrel or vessel, much larger than the receiver; and which by the action of a smaller pump, placed on a lower level, might be alternately filled with water and emptied so as in a few operations to complete the exhaustion, very nearly.
Thus, in [fig. 2] of [Plate 48], A is a receiver, B is a large vessel that can be filled with water from the tub C below; and D is the pump, worked by the handle E. It is a common water pump, (so much the readier adopted, as requiring little care in the execution.) The question was to make this pump alternately fill and empty the vessel B. Adverting first to the filling, a c are two cocks, having each a side-passage for the water; and these passages are now so placed, as by working the pump we suck water out of the tub C, and throw it into the vessel B, through the valve b;—by which means all its air is driven out through the lateral valve e. When this is done, the cocks c d (which are so made as to be worked by the same mover) are turned into a new position, which opens the pipe p to the pump D, and that q to the returning spout r; by which means the water is drawn from the vessel B, and thrown into the tub C: so that the air is again drawn out of the receiver A, through the inverted valve s, into the vessel B, and another degree of exhaustion occasioned. This being done, the cocks are again put into their present position; the air expelled by the water through the valve e as before, and a new stroke prepared. It is scarcely needful to add, that if the vessel B contained ten times as much volume as the receiver A, the exhaustion of the latter at each emptying of the vessel B would follow this ratio—1⁄11, 1⁄121, 1⁄1331, &c. thus approaching by rapid degrees to a perfect vacuum. The water, or liquid, used for this purpose would of course be as perfectly purged of air, as possible.