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II. § 27.] CONTINUITY OF PITCH. 57
diate neighbourhood. Presently a deep musical sound begins to be heard, which, as the velocity of rotation increases, constantly rises in pitch. The acuteness of the sound thus obtainable depends solely on the speed to which we can urge the instrument, and is therefore limited only by the driving-power at our command. The rise of pitch in this experiment is perfectly continuous, that is to say, the sound of the Syren, in passing from a graver to a more acute note, goes through every possible intermediate degree of pitch. It is important that we should familiarise ourselves with this conception of the continuity of the scale of pitch, because in the instrument from which our ideas on this subject are usually obtained—the pianoforte—the pitch alters discontinuously, i.e. by a series of jumps of half a Tone each, and we are thus tempted to ignore the intervening degrees of pitch, or even to suppose them non-existent. The more perfect musical instruments, such as the human voice or the violin, are as capable as the Syren of passing through all degrees of pitch from one note to another in the way called 'portamento' or 'slurring.'
It is clear from the nature of the Syren's construction that the only change which can take place during the rise in pitch is the increased number of impulses communicated to, and therefore of vibrations