263. =Difference Between a Musical Sound and a Noise.=--The difference between a musical sound and a noise is very analogous to the difference between a crystal and the same substance destitute of the crystalline arrangement. In both there are vibrations, but in the musical sound they have perfect regularity, while in a noise the vibrations are irregular, and there is confusion. Indeed so regular are the vibrations of musical sounds that the rules and principles of music have all the rigid exactness of mathematics. 264. =How Different Notes are Produced.=--The quicker is the vibration the higher is the note. Thus a short and small string on a violin or in a piano gives a higher note than a long and large string, because its vibrations are quicker. The tension of the string also has an influence, the note being raised by increasing the tension. In tuning a violin the right pitch is given to each string by lessening or increasing the tension by means of the screws to which the strings are attached. In playing upon it various notes are made upon each string by shortening the vibrating portion more or less by pressure of the finger. [Illustration: Fig. 190.] In wind instruments the note depends on the length and size of the column of air contained in them. This may be illustrated by an organ-pipe, Fig. 190 (p. 203). It is one of the pipes of what is called the flute-stop. It is constructed very much like a boy's willow whistle. The air from the bellows of the organ enters at P, and causes a vibration of the whole column of air in the pipe, the sound issuing at _t_. In the upper end is a movable plug, _s_, by which, in tuning, the note of the pipe is regulated. If the note be too grave this plug is pushed downward, so as to shorten the column of air. It is from difference in rapidity of vibration that a large bell gives a graver note than a small one. So, too, when musical sounds are produced by passing the moistened fingers over the edges of glass vessels, the larger the vessel the graver is its note. A tumbler will give a graver note than a wine-glass.