* Lift her ponderous waves. l. 346. The invention of the pump is of very antient date, being ascribed to one Ctesebes an Athenian, whence it was called by the Latins machina Ctesebiana; but it was long before it was known that the ascent of the piston lifted the superincumbent column of the atmosphere, and that then the pressure of the surrounding air on the surface of the well below forced the water up into the vacuum, and that on that account in the common lifting pump the water would rise only about thirty-five feet, as the weight of such a column of water was in general an equipoise to the surrounding atmosphere. The foamy appearance of water, when the pressure of the air over it is diminished, is owing to the expansion and escape of the air previously dissolved by it, or existing in its pores. When a child first sucks it only presses or champs the teat, as observed by the great Harvey, but afterwards it learns to make an incipient vacuum in its mouth, and acts by removing the pressure of the atmo∣sphere from the nipple, like a pump.
[ return to text ]