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I. § 3.] VELOCITY OF SOUND. 5
we confine our attention to sounds originating at but small distances from us, their passage through the intervening space appears instantaneous. If, however, a gun is fired at a considerable distance, the flash is seen before the report is heard—a proof that an appreciable interval of time is occupied by the transmission of the sound. The occurrence of an echo, in a position where we can measure the distance passed over by the sound in travelling from the position where it is produced to that where it rebounds, gives us the means of measuring the velocity of Sound; since we can, by direct observation, ascertain how long a time is spent on the out-and-hoine journey. The following easy experiment gives a near approximation to the actual velocity of Sound —in fact a much closer one than the rough nature of the observation would have led one to expect. In the North cloister of Trinity College, Cambridge, there is an unusually distinct echo from the wall at its eastern extremity. Standing near the opposite end of the cloister, I clapped my hands rhythmically at a rate such that the strokes and echoes were heard alternately at equal intervals of time. A friend at my side, watch in hand, counted the number of strokes and echoes. The result was that there were 76 in half a minute, i.e. 38 strokes and 38 echoes. A little consideration will show that the sound traversed the |
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