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ses are not generated by the pendulum." And finally, Daniell says: "A vibrating body, _before it can act_ as a sounding body, must produce alternate compressions and rarefactions in the air, and these must be well marked. If, however, the vibrating body be so small that at each oscillation the surrounding air has time to _flow round_ it, there is at every oscillation a local rearrangement--a local flow and reflow of the air; but the air at a distance is almost wholly unaffected by this." Now, as Prof. Carter has shown by experiment that a tuning fork _while still sounding_ had only an amplitude of swing of 1/17000 of an inch, and only traveled an aggregate distance of 1/33 of an inch in one second, or one inch in 33 seconds, surely such a motion is neither "swift," "fast," nor "vehement," and is unquestionably much "slower" than the motion of a pendulum. We have only to consider one forward motion of the prong, and if that motion cannot condense the air, then no wave can be produced; for after a prong has advanced and stopped moving (no matter for how short a time), if it has not compressed the air, its return motion (on the same side) cannot do anything toward making a compression. If one such motion of 1/17000 of an inch in 1/512 of a second cannot compress the air, then the remaining motions cannot. There is unquestionably a "union limit" between mobility and compressibility, and unless this limit is passed, mobility holds sway and prevents condensation or compression of the air; but when this limit is passed by the exercise of sufficient energy, then compression of the air results. Just imagine the finger to be moved through the air at a velocity of one foot in one hour; is it possible that any scientist who considers the problem in connection with the mobility of the air, could risk his reputation by saying that the air would be compressed? Heretofore it was supposed that a praeong of a tuning fork was traveling _fast_ because it vibrated so many times in a second, never stopping to think that its velocity of motion was entirely dependent upon the distance it traveled. At the start the prong travels 1/20 of an inch, but in a short time, _while still sounding_, the distance is reduced to 1/17000 of an inch. While the first motion was quite fast, about 25 inches in a second, the last motion was only about 1/33 of an inch in the same time, and is consequently 825 times slower motion. The momentum of the prong, the amou
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