To give one an idea of the differences produced by different rates of vibration, let us imagine a Mass of Iron, shaped like a great "Top," capable of being impelled to "spin" at a constantly increasing rate of speed, by some Mighty Will. At first it is seen as a slowly spinning Top, manifesting nothing but slow motion, to our senses. Now, imagine our Top spinning at a rate doubling each second. The first second the Top spins at the rate of two revolutions per second. We notice no change, except that we can see the movement. The next second the revolutions are doubled to four per second. Then, doubling each second, we have, respectively, revolutions of eight per second, then sixteen, and then in the fifth second thirty-two per second. Then we begin to notice a change. When the revolutions reach thirty-two per second the friction of the moving Top on the air causes it to give forth a very low, deep, bass note of sound. This note is like a low, deep "hum," and is the lowest possible of perception by the human hearing, although it is possible that some of the lower forms of life may be conscious of still lower vibrations. The sixth second the revolutions reach sixty-four, and the low note has grown much higher in the scale. The seventh second records a rate of 128, and the note has correspondingly increased. Then, as the seconds pass, we have, successively, 256, 512, 1,024, 2,048, 4,096, 8,192, 16,384, 32,768, the latter in the fifteenth second, and representing the highest note recognizable by the human ear, although it is believed that some of the lower animals may recognize sounds too acute for our sense of hearing. During this increase in revolutions from the fifth second to the fifteenth, the sound-note has risen rapidly in the scale from the low sullen "hum," on through the notes of the musical scale, and beyond the range of instruments, until the shrillness becomes so intense as to be almost unbearable, and finally terminating in a shrill, piercing shriek like the "squeak" of the bat, only long-drawn out. Then from the termination of the sound (by reason of the rate of vibration having become too high) silence reigns for thirty seconds--absolute silence, in spite of the rapidly increasing rate of vibrations, in fact, because of it.