ATMOSPHERIC PRESSURE.

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above half the atmosphere, where the air in the flask has to bear only half the superincumbent pressure that it bore at the sea-level. Now it is a curious law, peculiar to gaseous matter, that its density is commonly proportional to the pressure that confines it, that is to say, by doubling this pressure we compress any air into half its former bulk (as in the above instance (4) of a diving bell under 34 feet of water); and on the other hand, on removing half the ordinary pressure from any air, it expands to twice its ordinary bulk; so that there appears no limit to the space which any quantity, however small, would fill, if relieved of all pressure. We shall return to this important law presently; but meanwhile it must not be supposed, because an elevation of 3 miles leaves one-half of the atmosphere below us, that we should reach the limits of its existence at double that height, or 7 miles. At that height (supposing it attainable) we should still have one-fourth of the atmosphere above us, and 100 cubic inches of air from the sea-level would expand into 400, because the upper parts of the atmosphere, having less weight to bear than the lower parts, expand into far greater bulk; so that not under an elevation of 45 miles is the atmosphere supposed to be limited by the coast line of eternal space.

Thus the aerial ocean is not, like the sea, of nearly the same density throughout its depth, but gets thinner and thinner from the bottom upwards, so much so that the first 3 miles above the earth's surface contain as much air as all the remaining 41 or 42 miles. The cause for this is, that the air at the level of the sea has to bear the weight of the whole mass of atmosphere above it, which of course acts as a powerful mechanical force in increasing the density, and consequently the pressure of the lower strata.

Fig. 3.

The pressure of the atmosphere at the sea-level can Two hollow be estimated by a simple contrivance. hemispheres of brass, Fig. 3, fitting together with smooth edges, are placed in contact; the lower hemisphere is furnished with a short tube opening into it, and this tube can be opened or closed at pleasure by means of a stop-cock. On screwing the tube into an exhausting syringe, and placing the two hemispheres together, the air can be withdrawn from the hollow sphere thus formed, and on turning the stop-cock, before removing the apparatus from the syringe, the air is prevented from entering. A handle may now be screwed to the short tube, and if two persons pull in opposite directions they will be unable to separate the hemispheres. On turning the stop-cock, however, the air rushes in, and the hemispheres fall asunder by their own weight.

Now the force which binds these two hemispheres together is the pressure of the atmosphere, which may easily be calculated by suspending them, when exhausted, by the upper handle, and adding weights to the lower handle. Suppose the sphere to be 6 inches in diameter, its section through the centre will be about 29 square inches; and, supposing the vacuum to be perfect, a weight of 420 lbs. will be required to separate the hemispheres. Now 420 about 14 lbs., the amount of atmospheric pressure upon 1 square inch of surface.

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There are many other methods of proving the important fact, that the weight or pressure of the air is equal to between 14 and 15 lbs. on every square inch of surface at the level of the sea; or, in other words,

ATMOSPHERIC PRESSURE.

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a column of atmospheric air one inch square, resting on any surface at the sea-level, and extending to the top of the atmosphere, weighs between 14 and 15 lbs. This will be more clearly seen when we come to speak of the barometer; but we may here anticipate the surprise of the reader who approaches this subject for the first time. He may regard such results as these as scientific curiosities, in which he is in no way concerned; but a moment's reflection will convince him that the same force which held the hollow hemispheres together, is present and active, as well for animals as for inorganic matter. If it can be proved that a column of air one inch square and about 45 miles high weighs about 15 lbs., it is evident that this pressure must be as true for a square yard, or a square mile of surface, as for a square inch; the only difference is in extent; for, if we wish to know the pressure on a square yard, or mile, we must calculate the number of square inches in such a surface, and multiply this number by 15, and the product will give the atmospheric pressure in pounds on the larger surface. But since we have seen that air is impenetrable, and that our bodies displace it, it must be evident that this pressure is also exerted on the surface of our bodies, as on that of the earth on which the air rests, the pressure of a fluid on any surface immersed in it being (we must remind him) exactly equal, whether the surface be horizontal, vertical, overhanging, or even facing downwards like a ceiling (see Rudimentary Mechanics). We dwell on the floor of an ocean of air 45 miles deep, and are as much subject to its pressure as the bodies of fishes, which inhabit the floor of the liquid ocean, are to the column of water above them. In order, therefore, to calculate the amount of atmospheric pressure on our bodies, we must ascertain the number of square inches on their surface,

and multiply this number by 14. In this way it will be found, that the atmospheric pressure upon the body of a man of ordinary stature amounts to no less than 33,600 lbs., or about 15 tons! Why, then, it may be asked, is he not crushed to death, instead of being entirely insensible of this enormous pressure? A few examples will explain this. There are many delicate and fragile animals which live at great depths in the sea, often from 2000 to 3000 feet below its surface. These creatures, therefore, have to sustain the pressure of a column of water of that height, a pressure of from 60 to 90 times greater than that of the atmosphere upon our bodies. Yet these animals are not crushed; they move about with perfect ease, under circumstances still more surprising than those under which we live. And the reason is, that this hydrostatic pressure is equal on all sides; the bodies of these animals are equally pressed above, below, and around, and the fluids within the animal are also either of similar density, or they are nearly incompressible, so that all these different pressures counterbalance each other. In the same manner the fluid atmosphere presses equally in all directions, and the human body immersed in it may be compared to a sponge plunged into deep water; it is not crushed, because the water fills the cavities of the sponge, and also surrounds it entirely. In like manner our bodies, and even our bones, are filled either with liquids capable of sustaining pressure, or with air of the same density as the external air, so that the outward is counteracted by the inward pressure. Now let us see what are the consequences of removing this pressure. Some fishes, which live at great depths in the sea, are provided with swimming bladders, or little bags full of air. On raising them to the surface, the water pressure is removed, and the bladder expands

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to such a degree as to kill the creatures instantly. In like manner, if we were raised towards the surface of our aerial ocean, our bodies would swell, and probably burst. We become painfully sensible of a partial effect of this kind, by partly removing the external pressure from a portion of the skin, as in the operation of cupping. The cupper drives out the greater portion of the air from the cupping glass, by holding it over the flame of a spirit lamp, and then suddenly claps the glass on the skin, which has been previously cut by a number of small lancets. The cup adheres by the pressure of the air on the outside, while the flesh beneath the glass, being relieved from pressure, expands, and forms a projection within the cup. The blood vessels beneath the incised portion of the skin, being also relieved from pressure, discharge their contents into the rarefied space formed in the cup.

8. We have thus far illustrated certain properties which air enjoys in common with solids, namely, impenetrability, weight, inertia, momentum, and fluid pressure in all directions. We have also slightly noticed its compressibility, and its elasticity when compressed. This last property requires a more extended notice, for, although common to all matter, it is so much more obvious in airs and gases as to be sometimes regarded as their distinguishing feature, and to gain for them the somewhat ambiguous title of elastic fluids.

Airs and gases are so different in structure from solids and liquids, that it seems difficult to suppose them to be regulated by the same mechanical laws. The atoms or particles of solids are held together by an attractive force called cohesion, which differs in different solids, as is evident from the various degrees of force required to crush or grind them to powder.