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I. § 20.] ATMOSPHERIC PRESSURE. 41
When a pulse of condensation is traversing the section, its parts will be more dense, and when a pulse of rarefaction is traversing it, less dense, than they would be were the column transmitting no waves at all, and its separate particles, therefore, absolutely at rest. Let the column with which we have been dealing be the portion of atmospheric air enclosed within a tube of uniform bore. The phenomena just described will then be exactly those which accompany the passage of a sound from one end of the tube to the other. It remains to examine the mechanical cause to which these phenomena are due.
Atmospheric air in its ordinary condition exerts a certain pressure on all objects in contact with it. This pressure is adequate to support a vertical column of mercury about 30 inches high, as we know by the common barometer. Fig. 18 represents a section of a tube closed at one end, with a movable piston fitting air-tight into the other. In (0) the air on both sides of the piston is in the ordinary atmospheric condition, so that the pressure on the right face of the piston is counteracted by an exactly equal and opposite pressure on its left face.
In (l) the piston has been moved inwards, so as to compress the air on the right of it. That on its left, being in free communication with the external