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Let a small piece of cork be placed floating on the surface of water in a basin or other vessel, and an empty glass goblet be inverted over the cork, so that its edge just meets the water. A portion of air will then be confined in the goblet, and detached from the remainder of the atmosphere. If the goblet be now pressed downwards, so as to be entirely immersed, it will be observed, that the water will not fill it, being excluded by the impenetrability of the air inclosed in it. This experiment, therefore, is decisive of the fact, that air, one of the most subtle and attenuated substances we know of, possesses the quality of impenetrability. It absolutely excludes any other body from the space which it occupies at any given moment.

But although the water does not fill the goblet, yet if the position of the cork which floats upon its surface be noticed, it will be found that the level of the water within has risen above its edge or rim. In fact, the water has partially filled the goblet, and the air has been forced to contract its dimensions. This effect is produced by the pressure of the incumbent water forcing the surface in the goblet against the air, which yields until it is so far compressed that it acquires a force able to withstand this pressure. Thus it appears that air is capable of being reduced in its dimensions by mechanical pressure, independently of the agency of heat. It is compressible.