“Is that a pump?” asked Tom: “I should certainly never have guessed what you intended to represent.”
“It is not a perspective drawing, my dear, but a representation of the different parts as they would appear, were it possible to cut the pump in halves, from top to bottom, without disturbing any of its arrangements. A drawing of this kind, which is frequently used for the sake of explanation, is termed a section.”
Mr. Seymour here took an apple from his pocket, and having cut it in two, observed that the surfaces thus exposed presented sections of the fruit. This illustration was understood by all present, and Mr. Seymour continued, “I have here, then, a section of the common household pump. A B is the cylinder or barrel; P the air-tight piston which moves or works within it, by means of the rod; Q is the ‘suction,’ or ‘feeding pipe,’ descending into a well, or any other reservoir; S the valve, or little door, at the bottom of the barrel, covering the top of the feeding pipe; and there is a similar valve in the piston, both of which, opening upwards, admit the water to rise through them, but prevent its returning. As this part of the apparatus is no less ingenious than it is important, I will sketch the valve, or clack, as it is termed by the engineer, on a larger scale.”
Their father then chalked the annexed figure; from which its construction was rendered perfectly intelligible to the children.
Mr. Seymour proceeded: “When the pump is in a state of inaction, the two valves are closed by their own weight; but, on drawing up the piston P, from the bottom to the top of the barrel, the column of air, which rested upon it, is raised, and a vacuum is produced between the piston and the lower valve, S; the air beneath this valve, which is immediately over the surface of the water, consequently expands, and forces its way through it; the water then ascends into the pump. A few strokes of the handle totally exclude the air from the body of the pump, and fill it with water: which, having passed through both valves, runs out at the spout.”
“I understand how water may be thus raised to the elevation of 32 feet, but I have yet to learn the manner in which it can be raised above that distance,” said Louisa.
“It is undoubtedly true that, if the distance from the surface of the water to the valve in the piston exceed 32 feet, water can never be forced into the barrel; but you will readily perceive that, when once the water has passed the piston valve, it is no longer the pressure of the air which causes it to ascend; after that period, it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom; and water, having been so raised, cannot fall back again, in consequence of the valve, which is kept closed by its pressure. All, therefore, that is necessary, is to keep the working barrel within the limits of atmospheric pressure; we have then only to fix a continued straight pipe to the top of the barrel, and to lengthen the piston rod in the same proportion, and the water will continue to rise at each successive stroke of the pump, until at length it will flow over the top of the pipe, or through a spout inserted in any part of its side. The common pump, therefore, is properly called the sucking and lifting pump.”
The party expressed themselves fully satisfied; and Tom enquired who invented the machine.
“It is an instrument of great antiquity,” replied his father: “its invention is generally ascribed to Ctesebes of Alexandria, who lived about 120 years before Christ; but the principle of its action was not understood for ages after its invention. The ancients entertained a belief that ‘Nature abhorred a vacuum;’ and they imagined that, when the piston ascended, the water immediately rushed forward to prevent the occurrence of this much dreaded vacuum. In the seventeenth century a pump was constructed at Florence, by which it was attempted to raise water from a well to a very considerable altitude, but it was found that no exertion of this machine could be made to raise it above 32 feet from its level. This unexpected embarrassment greatly puzzled the engineer, until Galileo suggested that the pressure on the water below must cause its ascent into the pump, and that, according to this theory, when it had risen 32 feet, its pressure became equivalent to that of the atmosphere, and could, therefore, not rise any higher; and as they did not, at that time, understand the construction of the piston valve, the design was abandoned. It is now time to conclude your lesson. To-morrow I hope we shall be able to enter upon the subject of The Kite.”