produced between the piston and the lower valve, c; the air beneath this valve, which is immediately over the surface of the water, consequently expands, and forces its way through the valve; the water then ascends into the pump, being, in fact, pushed up by the weight of the atmosphere pressing on the surface of the water in the well; for this pressure is transmitted in all directions throughout the liquid. A few strokes of the handle suffice to remove 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 in the well to the valve in the piston exceed 32 feet, water can never ascend above the piston-valve; but you will readily perceive that when once the water has passed the pistonvalve, it is no longer the pressure of the air which causes it to ascend: after that 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, in consequence of the valve, which is kept closed by its pressure, fall back again. All that is necessary, therefore, 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 pistonrod 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 may, therefore, be described as at once a suction and a lifting pump.”

The party expressed themselves fully satisfied with these explanations, and Tom inquired who invented the machine.

"It is an instrument of great antiquity," said Mr. Goodenough; "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."

6

"I am not quite clear," said Mr. Seymour, "that the phrase 'nature abhors a vacuum' was ever seriously advanced as an explanation of these facts. Might not this have been merely a

TORRICELLI'S EXPERIMENT.

289

compendious expression for summarizing the facts? 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 33 feet from its level. This unexpected embarrassment greatly puzzled the pumpmaker, and Galileo was asked for an explanation of the circumstance. It is said that his reply was that Nature's abhorrence of a vacuum did not extend to greater distances than 33 feet, and that her efforts to fill the vacuum ceased at that point. Some have supposed that if Galileo really did make this answer, he did it sarcastically; but it is more probable that had he at the time been prepared with a more satisfactory solution, he would have advanced it, and that, failing this, he merely extended the hypothetical expression which had, up to that time, appeared consistent with the facts, by modifying it so as to include the new fact; thus, 'Nature abhors a vacuum-up to 33 feet.' It was Galileo's pupil, Torricelli, who first demonstrated that the pressure of the air on the water below is the cause of the liquid rising in the pump; and that as, when it has risen 32 feet, its pressure becomes equal to that of the atmosphere, it cannot rise any higher. The experiment by which Torricelli proved this was not only a conclusive one, but it provided the world with that highly useful instrument, the barometer; for the 'Torricellian tube,' as it was called, was really nothing more than a simple form of the barometer. This famous experiment consisted in filling with quicksilverwhich is about thirteen-and-a-half times heavier than water a strong glass tube about a yard long, and closed at one end. The open end of the tube was covered to prevent the escape of the quicksilver, until the tube had been inverted and its mouth brought below the surface of some more quicksilver in an open basin, when the end was left open, so that the liquid in the tube communicated freely with that in the basin. But here is a little sketch (Fig. 89) by which you will at once see what I mean. Well, what then happens with such an arrangement is

30.inches.

Fig. 89.

this: when the submerged end of the tube is opened, some of the quicksilver flows out, and unites with that in the basin, but only so much as suffices to bring the level of the quicksilver in the tube to about 30 inches vertically above that of the liquid in the basin. Thus the atmospheric pressure sustains a vertical column of mercury shorter than that of the column of-water, in the same proportion as quicksilver is heavier than water. I hope before long to explain to you at large the uses of the barometer; it may be sufficient for the present that you observe that the height of the mercury in the tube will change when the pressure of the atmosphere changes. When that becomes greater, more mercury is forced up into the tube; when that is lessened, it is able to counteract the weight of only a shorter column of mercury, and consequently the height of the column in the tube is diminished by some of the liquid leaving the tube to enter the basin."

"But I cannot understand," said Louisa, "how the pressure of the mercury can balance the pressure of the atmosphere; for the former is exerted only over the narrow width of the tube, while the atmosphere presses upon the greatly more extended surface of the liquid in the vessel."

"It is a property of fluids," replied Mr. Seymour, "that a pressure applied to any part is transmitted throughout the mass, so that, on equal areas at every part, the same pressure is produced. Thus the small quantity of water in the spout of a teapot balances the larger quantity within the vessel, and the two portions of the liquid have always the same level. But it is now time to conclude your lesson: to-morrow I hope we shall be able to enter upon the subject of the kite."

THE

HE children were summoned into the library, and informed by their father that he was at leisure to explain the philosophy of the kite; a subject with which Tom had repeatedly expressed some impatience to become acquainted.

"It is a beautiful day," exclaimed the boy, joyously; "and there is such a delightful breeze, that I should really call it a complete kite-day."

"Gently, my fine fellow," replied Mr. Seymour: "the bird must be fledged ere it can fly. We have not, as yet, any kite, for you know that the one you possess is shattered beyond the possibility of repair."

"True, papa; but could not Robert just step into the village

and buy one? I saw several kites in the shop of Peg Robson yesterday."

"I do not doubt it, my boy; but the kites which are to be. found in the toy-shop are made to sell rather than to fly, and to raise the wind for the benefit of the vendor, rather than to be raised by it for the amusement of the purchaser: we must, therefore, construct one for ourselves; and see, I have accordingly prepared all the necessary materials for the purpose. I have here, as you perceive, a straight lath of deal, about threequarters of an inch wide, and less than a quarter of an inch thick, and about four feet in length; this is quite ready for forming the back-bone of the kite: and now for the bow. The cooper has complied with my directions, and sent an unbent hoop as free as possible from knots: you observe that it is about the same length as the lath, but it will be necessary to pare it down a little at each end, in order to make it bend more readily to the required shape."

This having been accomplished, Mr. Seymour proceeded to form the framework of the kite in the following manner: He

first ascertained the central point of the hoop by balancing it on his forefinger; he then, by means of string, attached the hoop at that point to the lath, about an inch and a half from its end, as shown at a (Fig. 90); he next cut a notch in each end of the hoop, or bow, c D, and having fixed the string in the notch c, he drew it through another, B, previously cut in the other end of the lath, and carried it to the opposite extremity of the bow, D. The skeleton now presented the usual form of the kite. The next point, therefore, was to ascertain whether ! Fig. 90. the two sides of the bow were in equilibrium, and this he determined by balancing the lath on the finger, and observing whether it remained horizontal or dipped on one side. This adjustment having been accomplished, Mr. Seymour next continued the string from D across the skeleton to the opposite notch c, giving it in its way one turn round the lath at E; from c it was carried to A, and wound round the top of the lath, and then again fastened at D; from D it was extended to a point F, about midway down the lath, and having been there secured, was finally carried again to the notch c,

B