But when by the air-pump we remove the pressure from one side of a body, and still allow it to be exerted on the other, we see

at once abundant evidence of the intensity of this force. Thus, if we take a jar, Fig 17, open at both ends, and having placed it on the pump-plate, lay the palm of the hand on the mouth of it on exhausting the air, the hand is pressed in firm contact with the jar, so that it cannot be lifted without the exertion of a very considerable force.

Fig. 17.

Fig. 18.

In the same way, if we tie over a jar a piece of bladder, and allow it to dry, it assumes, of course, a perfect horizontal position; but on exhausting the air within very slightly, it becomes deeply depressed, and is soon burst inward with a loud explosion. This simple instance illustrates, in a very satisfactory way, the mode in which the pressure of the air is thus rendered obvious; for so long as the jar was not exhausted, and had air in its interior, the downward pressure of the atmosphere could not force the bladder inward, nor disturb its position in any manner: for any such disturbance to take place, the pressure must overcome the elastic force of the air within, which resists it, pressing equally in the opposite way. But on the removal of the air from the interior, the pressure above is no longer antagonized, and it takes effect at once by crushing the bladder.

CHAPTER V.

THE PRESSURE OF THE AIR.

The Magdeburgh Hemisphere-Water supported by Air-The Pneumatic Trough-Description of the Barometer-Cause of its Action-Different kinds of Barometers-Measurement of Accessible Heights.

a

MANY beautiful experiments establish the fact that the atmosphere presses, not only in the downward direction, but also in every other way. Thus, if we take a pair of hollow brass hemispheres, a b, Fig. 19, which fit together without leakage, by means of a flange, and exhaust the air from their interior through a stop-cock affixed to one of them, it will be found that they cannot be pulled apart, except by the exertion of a very great force. [In order to make the contact more perfect, the hedges of the hemispheres are rubbed with grease previous to the air being exhausted.] Now it does not matter whether the handles of these hemispheres are held in the position represented in the figure, or turned a quarter way round, or set at any angle to the horizon, they adhere with equal force together; and the same power which is required to pull them asunder in

Fig. 19.

C

the vertical direction must also be exerted in all others. This, therefore, proves that the pressure of the air takes effect equally in every direction, whether upward or downward, or laterally.

[This experiment merits recollection, because it was one of the first which drew attention to the material nature and properties of the air, and it astonished the world. Otto de Guericke, of Magdeburgh, the inventor, had hemispheres made a foot in diameter; and once when he exhausted them, on the occasion of a public exhibition, six coach-horses of the Emperor were unable to pull them asunder.-Arnott's "Elements of Physics."]

[Two small hemispheres of copper were exhausted and placed between four strong dray-horses; but they could not, separate them although dragging in opposite directions for about half an hour.]

Take a wine-glass, and having filled it entirely with water, place over its mouth a slip of writing-paper. If the wine-glass be inverted, it will be seen that the fluid is supported, the paper neither dropping off nor the water flowing out. This remarkable result illustrates the doctrine of the upward pres

F

sure of the air. Nor does it even require that a piece of paper should be used, provided the glass has the proper form. Thus, let there be a bottle, a, Fig. 20, in the bottom of which there is a large aperture, b. If the bottle be filled with water, and its mouth closed by the finger, the water will not flow out, but remain suspended. And that this result

is due to the upward pressure of the air is proved by moving the finger a little on one side, so as to let the air exert its pressure on the top as well as the bottom of the water, which immediately flows out.

Fig. 20. If we take a jar, a, Fig. 21, and having filled it full of water, invert it, as is represented, in a reservoir, or trough; for the reason explained in reference to Fig. 19, the water will remain suspended in the jar. Such an arrangement forms the pneumatic trough of chemists. It enables them to collect the various gases without intermixture with atmospheric air; for if a pipe, or tube, through which such a gas is coming be depressed beneath the mouth of the jar, a, so that the bubbles may rise into the jar, they will displace the water, and be collected in the upper part without any admixture.

If in this experiment we use mercury instead of water, the same phenomenon ensues-the mercury being supported by the pressure of air. Now, it might

be inquired, as the atmosphere only extends to a certain altitude, and therefore presses with a weight which, though great, must necessarily be limited, whether that pressure could sustain a column of mercury of an unlimited length? If we take a jar a yard in length, and fill it with mercury, and invert it in a trough, it will be seen that the mercury is not supported, but that it settles from the top and descends until it reaches a point which is about thirty inches above the level of the mercury in the trough. Of course, as nothing has been admitted, there must be a vacant space or vacuum between the top of the mercury and the top of the jar.

Fig. 21.

This experiment, which, as we are soon to see, is a very important one, is

commonly made with a tube, a b, Fig. 22, instead of a jar-the tube being more manageable, and containing less mercury. It should be at least thirty-two inches long, and being filled with quicksilver, may be inverted in a shallow dish, containing the same metal, c. It is convenient to place at one side of the tube a scale, d, divided into inches, these inches being counted from the level of the mercury in the dish, c. Such an instrument is called a Baro

d

meter, or measurer of the pressure of the air.

Let us briefly investigate the agencies which operate in the case of this instrument. If, having closed the mouth of the tube, b, with the finger, we lift it out of the dish, c, it will be found that we must exert a considerable degree of force in order to sustain the column of mercury, which presses against the finger with its whole weight, and tends to push it away. Consequently, the mercury is continually exerting

Fig. 22. a tendency to flow out, and therefore two forces are in operation; on the one hand, the weight of the mercury attempting to flow out of the tube into the dish; and on the other, the weight or pressure of the atmosphere attempting to push the mercury up in the tube. If the pressure of the air were greater, it would push the mercury higher; if less, the mercury would flow out to a corresponding extent. Thus, the length of the mercury column equilibrates the pressure of the air, and we therefore say that the atmospheric pressure is equal to so many inches of mercury.

That the whole thing depends on the pressure of the air may be beautifully proved by putting the barometer under a tall air-pump receiver, as represented in Fig. 23, and exhausting. As the pressure of the air is reduced, the mercurial column falls; and if it were possible to make a perfect vacuum by such means, the mercury would sink in the tube to its level in the dish. On re-admitting the air the mercury rises again, and when the original pressure is regained it stands at the original level.

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a

There are many different forms of barometers, such as the straight, the syphon, &c., but the principle of all is the same. The scale must uniformly commence at the level of the mercury in the reservoir. Now, it is plain that this level changes with the height of the column; for if the metal flows out of the tube, it raises the level in the reservoir, and In every perfect barometer, means, therefore, should be had to adjust the beginning of the scale to the level for the time being. In some barometers, as in that represented in Fig. 24, this is done by having the mercury in a cistern with a moveable Fig. 24. bottom, and by turning the screw V, the level can be precisely adjusted to that of the ivory point, a.

Fig 23. vice versa.

A barometer kept in the same place undergoes variations of altitude, some of which are regular, and others irregular. The former, which depend on diurnal tides in the atmosphere, analogous to tides in the sea, occur about

the same time of the day-the greatest depression being commonly about four in the morning and evening, and the greatest elevation about ten in the morning and night. In summer, however, they occur an hour or two earlier in the morning, and as much later at night. The irregular changes depend on meteorological causes, and are not reduced as yet to any determinate laws. In amount they are much more extensive than the former, extending from the twenty-seventh to more than the thirtieth inch, while those are limited to about the tenth of an inch.

A very valuable application of the barometer is for the determination of accessible heights. The principle upon which this depends is simplethe barometer necessarily standing at a lower point as it is carried to a higher position. In practice it is more complicated; and to obtain exact results various methods have been given by Laplace, Bailly, Littrow,

and others.

[The pressure of the atmosphere varies according to the elevation above the level of the sea, and on this principle the height of mountains is esti mated. Supposing the density of the atmosphere to be uniform, a fall of one inch in the barometer would correspond to 11,065 inches, or 922 feet of air; but in order to make the calculation with accuracy, allowance must be made for the increasing rarety of the air, and for various other circumstances which are detailed in works on meteorology.-Turner's" Elements of Chemistry.”]

CHAPTER VI.

THE PRESSURE OF THE AIR.

Measure of the Force with which the Air presses-Different Modes of Estimating it-Experiments illustrating this Force-Elasticity of the AirExperimental Illustrations-The Condenser.

HAVING explained the cause, and illustrated the pressure of the air, we

proceed in the next place to determine its actual amount.

There are many ways in which this may be done. The following is simple: Take a pair of Magdeburgh hemispheres, the area of the section of which has been previously determined in square inches; exhaust them as perfectly as possible at the pump; and then, fastening the lower handle, a, to a firm support, hang the other, b, Fig. 25, to the hook of a steelyard, and move the weight until the hemispheres are pulled apart. It will be found that this commonly takes place when the weight is sufficient to overcome a pressure of fifteen pounds on every square inch.

a

Fig. 25.

This may serve as an elementary illustration, but there are other methods much more exact. Thus, by the barometer itself we may determine the

value of the pressure with precision. If we had a barometer which was exactly one square inch in section, and weighed the quantity of mercury it contained at any given time, it would give us the value of the atmospheric pressure on one square inch, because the weight of the mercury is equal to the pressure of the air. And by calculation we can, in like manner, obtain it from tubes of any diameter.

The phenomena of the barometer teach us that this pressure is not always the same, but it undergoes variations. It is commonly estimated at fifteen pounds on the square inch.

pro

[The pressure of the atmosphere being about fifteen pounds on every square inch, the surface of the whole globe sustains a pressure of 11,449,000,000 hundreds of millions of pounds. Shell-fish, which have the power of ducing a vacuum, adhere to the rocks by a pressure of fifteen pounds upon every square inch of contact.-Somerville's" Connection of the Physical Sciences."]

There are two other ways in which the value of the pressure of the air is stated. It is equal to a column of mercury thirty inches in length, or to a column of water thirty-four feet in length.

Fig. 26.

We are now able to understand the reason of the great effects to which the pressure of the air may give rise. In most instances these effects are neutralised by countervailing pressures. Thus, the body of a man of ordinary size has a surface of about two thousand square inches, the pressure upon which is equal to thirty thousand pounds. But this amazing force is entirely neutralised, because, as we have seen, the atmospheric pressure is equal in all directions-upward, downward, and laterally. All the cavities and the pores of the body are filled with air, which presses with an equal force. [What a fortunate thing it is that we are subjected to this apparent enormous pressure, for if this did not exist, the fluids that circulate within our bodies would be vapourised, and the surrounding parts crumble away.]

The following experiments may further illustrate the 'general principle of atmospheric pressure:

On a small flat plate, a, Fig. 26, furnished with a stop-cock, b, which terminates in a narrow pipe, c, let there be placed a tall receiver, from which the air is to be exhausted by the pump. The stop-cock, b, being closed, and the instrument being removed from the pump, b is to be opened, while the lower portion of its tube dips into a bowl of water. Under these circumstances the water is pressed up in a jet through c, and forms a fountain in

vacuo.

On the top of a receiver, Fig. 27, let there be cemented air-tight, a cup of wood, a, terminating in a cylindrical piece, b, the pores of which run lengthwise. Beneath this let there be placed a tall jar, c. Now, if the wooden cup be filled with quicksilver, the jar being previously placed on the pump, and exhaustion made, the metal will be

Fig. 27.