« السابقةمتابعة »
Magdeburg, more than 200 years ago, and hence the apparatus is called the Magdeburg hemispheres.
A bladder half filled with air, if placed beneath the receiver of an air-pump, will gradually swell as the exhaustion proceeds, and at last will burst with a loud report. A thin square phial, firmly corked, will be shivered to pieces in like manner. In the same situation, shrivelled fruit becomes plump, the flame of a candle goes out, a bell ceases to give forth any sound, and water boils almost before it is warm. Some have tried how a living animal would be affected by exhausting the air around it. The experiment is a cruel one, and one which no good boy would like to look at. A mouse, for example, placed beneath a receiver, suffers the greatest agony as the air becomes less and less dense, till at last the violent rupture of some of its blood vessels puts an end to its distress. There can be no doubt that the experimenter himself, if placed beside it, would share the same fate.
WATER-PUMPS. It seems impossible that water should flow upwards, and we naturally think that it never does so of its own accord. So far we think rightly. But neither is it of its own accord that it flows downwards; for it has no power of motion in itself, and, were there no force acting upon it, it could not move either the one way or the other. Gravity causes it in general to seek a lower level; but, on the other hand, there is no reason why, if impelled by any adequate force, it should not as readily ascend to a higher. We saw in a former lesson* that it rises in a close tube laid across a valley to the same level at which it enters the tube. In a common suction-pump, it is raised by the pressure of the atmosphere.
The suction-pump consists of a cylinder A B, similar in all respects to that of the air-pump; the tube or pipe BD
* Page 301.
is wider than the tube of the air-pump, and instead of communicating with a receiver, goes down into the well or reservoir containing the water to be raised. Into this pipe the water is admitted at D through a Tc. 47. number of small holes, so as to exclude any loating solids which might tend to choke up the pipe. Before the action of the pump commences, the water inside and outside the pipe will stand at the same level O. The piston is usually worked by a lever attached to its rod, and supported on a fulcrum above A. On raising the piston, the air in the pipe and lower part of the cylinder becomes rarefied, exactly as in the air-pump. Its pressure on the surface of the water in the pipe is thus diminished, and is no longer sufficient to balance the pressure of the atmosphere on the surface of the water outside. The water accordingly rises in the tube to restore equilibrium, on the same principle which causes the
el mercury in the tube of a barometer to stand higher than the mercury in the basin. As the exhaustion goes on, the water rises through the valve c into the cylinder, the lower part of which soon becomes filled. When the piston again descends, this water being prevented by the valve c from returning into the pipe, forces up the valve at m, and part of it escapes into the space above the piston. The piston, in its next ascent, carries this portion upwards along with it, and the vacuum thus left below is filled by a fresh supply from the pipe. This process goes on till the water above the piston reaches the spout i. Whenever any portion of the water rises above either vaive, it is clear that it cannot return, because both the valves open upwards.
The pressure of the atmosphere being equal, at the level of the sea, to the weight of a column of water about 34 feet high, the surface of the water under the piston can never, in any case, be more than 34 feet above the surface of the water in the well or reservoir below. In practice, however, we cannot procure a perfect vacuum, and therefore it is better that the line n, the upper limit of the piston's motion, should not be more than 28 feet above 0. It was an unsuccessful attempt to raise water in this way to a greater height than the atmospheric pressure will balance, that led to the discovery of the real cause of what we still commonly call suction. For suction is nothing else than the effect of producing a vacuum, or rarefaction of the air in any space, into which the pressure of the atmosphere immediately forces any fluid which has access to it. Thus we suck up water through a straw, and thus the infant draws milk from its mother's breast. The forcing-pump enables us to raise water to any height. FIG. 48. Its piston c differs from that of the suc
tion-pump in having no valve or aperture. Instead of this, there is a bent tube GE, communicating with the lower part of the cylinder, and provided with a valve at F, which opens upwards. The water rises through the tube B A into the cylinder, on the same principle as in the suction-pump. When the piston descends, it presses on the water in the cylinder, which, since the valve at A is closed against it, must force its way through the valve F. As soon as the pressure of the piston is removed, the valve F shuts, and prevents the return of any part of the water which has passed it. Meanwhile the piston is again
drawn up, leaving a vacuum, which is B filled by a fresh supply from the pipe.
This in its turn is forced upwards through the valve F, and by continuing the process the water may be made to rise in the tube G to any required height.
One of the most useful applications of this instrument is the fire-engine, by which so much valuable property has been saved. It is a combination of two forcing-pumps, throwing water copiously into a close vessel partly filled with air. The air, being much compressed, and therefore highly elastic, impels the water with great force along a flexible pipe or hose, from the mouth of which it gushes in a continuous stream upon the flames. Few better examples could be found of the benefits which mankind may derive from even a little knowledge of mechanical science.
QUESTIONS FOR EXAMINATION.
Name examples of gaseous bodies. What are the chief properties of air? Which is common to it with liquids ? Which are characteristic? Give exams ples of its impenetrability. What is wind? What property of air does the force of the wind illustrate? How is it proved that air has weight? What effect has it on the weight of bodies surrounded by it? State the relations between the volume of a mass of air and its elasticity. How is the atmosphere prevented from spreading outwards? Where is it densest? Show how the weight of the air may be estimated ? Explain the principle of the barometer. Why do we not feel the pressure of the air on our bodies? Of what value is the barometer as a weather-glass? For what other purpose is it used? What is the use of the air-pump? Explain its construction. What are valves used for? Explain the action of an air-pump with one cylinder. What is the usual form of the instrument? What are the Magdeburg hemispheres, and what do they illustrate? Why does water flow downwards? What is necessary to make it flow upwards? What force does so in the suction-pump? Explain the construction of such a pump, and show how it acts. How far can water be raised by suction? What is suction ? Explain the construction of a forcing-pump, and of a fire-engine,
QUARREL OF BRUTUS AND CASSIUS.
Cas. That you have wronged mel doth appear in this: You have condemned and noted Lucius Pélla, For taking bribes here of the Sàrdians; Wherein my létter, pràying on his side, Because I knéw the man, was slighted off.
Bru. You wronged yourself! to write in such a case,
Cas. In such a time as thís' it is not meet That every nice offence should bear its comment.
Bru. Let me tell you, Cassius, you yourself
Cas. I' an itching palm?
Bru. The name of Càssius hònours this corrúption,
Cas. Chástisement !
Bru. Remember Màrch, the 'Ides of March remember!
Cas. Brutus, bay not mè;
Bru. Go tò; you are not, Cassius.
Cas. Urge ine no more, I shall forget myself;
Bru. Awày, slight man !
Bru. Hèar me, for I will speak.
Cas. O ye gòds ! ye gods! must I endure all this?