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2 SENSATIONS AND THEIR CAUSES |
. § 1. |
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that some purely mechanical phenomena external to the ear will prove to be turned into the sensation we call Sound by a process carried on within that organ and the brain with which it is in direct communication. This mechanical agency, whatever may be its nature, is usually set going at a distance from the ear, and, to reach it, must traverse the intervening space. In doing so it can pass through solid and liquid as well as gaseous bodies. If one end of a felled tree is gently scratched with the point of a penknife, the sound is distinctly audible to a listener whose ear is pressed against the other end of the tree. When a couple of pebbles are knocked together under water, the sound of the blow reaches the ear after first passing through the intervening liquid. That Sound travels through the air is a matter of universal experience, and needs no illustration. In every case accessible to common observation where Sound passes from one point of space to another, it necessarily traverses matter, either in a solid, liquid or gaseous form. We may hence conjecture that the presence of a material medium of some kind is indispensable to the transmission of Sound. This important point can be readily brought to the test of experiment, as follows. Let a bell kept ringing by clockwork be placed under the receiver of an air-pump, and the air gradually exhausted. Provided |
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