Sometimes the air is so heavy and elastic as to support the mercury in the tube at the height of 31 inches nearly; at other times it is so light and unelastic, as to suffer it to fall as low as 28 inches. The difference between these two altitudes is three inches, that is, about 1-9th of the whole weight of the atmosphere. Our bodies, therefore, are sometimes pressed with a weight one-ninth more than at other times, that is, with about 3,360 pounds more weight at one time than another. This has considerable effect on our feelings, and consequently on our health, but we are apt to ascribe this effect to a wrong cause. When we feel ourselves dull and languid, we think it is owing to the air being too thick and heavy about us. But it is just the reverse: the air is then too light and thin, as is evident from the mercury’s sinking in the barometer, and its not bearing up the clouds: it is seldom dense enough at two miles height to bear them up.[65] The weight of the air is proved by its supporting the clouds and vapors which we so frequently see floating in it; in the same manner that the swimming of a piece of wood indicates the weight of the water which supports it. It may be remarked, says Mr. Parkes, that the Creator has endowed atmospheric air with the property of preserving its own _equilibrium_ at all times and in all places. Its elasticity is such, that, however it may be consumed by respiration or combustion, its place is immediately supplied with a new portion; and though by a mistaken policy the doors and windows of our habitations may be constructed so as to exclude it as much as possible, it will have admission; it forces its way through every crevice, and performs the most important office assigned it, in defiance of all our exertions. If the properties which are given to the different substances in nature, and the laws by which they are governed, be thus examined, we shall find them all tending to promote the welfare and felicity of every species of animated beings.