Mr. Seymour now proposed that they should proceed to consider the structure and operation of the pump. “I suppose,” said Louisa, “that the pump raises water in the same manner as the squirt.” “Exactly upon the same principle,” replied her father; “but the machinery is a little more complicated, since its object is not to force the water out of the pump at the same end of the pipe at which we draw it in. We will, however, proceed to the stableyard, and examine the pump; and do you, Tom, provide a piece of chalk, in order that I may make a sketch of some of its principal parts.” The party immediately proceeded; and, as they walked along, Mr. Seymour desired the children to remember that the weight of the atmosphere was estimated as being equal to that of fifteen pounds upon every square inch of surface; and that the moment the water arrived at such a height as to balance that pressure, it could ascend no higher: he added, that the altitude at which such a balance took place was about 32 or 33 feet above the surface. “If that be the case,” said Louisa, “the pump, of course, can never raise water from any well of greater depth than that which you state.” “Not without some additional contrivance, which I shall afterwards explain to you,” replied Mr. Seymour. The party had, by this time, arrived at the pump; its door was opened, and as much of the apparatus exhibited as could be conveniently exposed. Mr. Seymour then chalked the annexed sketch upon the stable door. [Illustration: Diagram of a water-pump’s mechanism.] “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.