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48 STEADY AND UNSTEADY SOUNDS [I. § 23.
of percussion, and then gradually dies away, diminution of loudness is the only change which occurs.
In the case of non-musical sounds variations of a different kind can be easily detected. In the howling of the wind the sound rises to a considerable degree of shrillness, then falls, then rises again, and so on. On parts of the coast where a shingly beach of considerable extent slopes down to the sea, a sound is heard in stormy weather which varies from the deep thundering roar of the great breakers, to the shrill tearing scream of the shingle dragged along by the retreating surf. Similar variations may be noticed in sounds of small intensity such as the rustling of leaves, the chirping of insects, and the like. The difference, then, between musical and non-musical sounds seems to lie in this, that the former are constant, while the latter are continually varying. The human voice can produce sounds of both classes. In singing a sustained note it remains quite steady, neither rising nor falling. Its conversational tone, on the other hand, is perpetually varying in height even within a single syllable; directly it ceases so to vary, its non-musical character disappears, and it becomes what is commonly called ' sing-song.'
We may then define a musical sound as a steady sound, a non-musical sound as an unsteady sound. It is true we may often be puzzled to say whether