[Illustration: FIG. 341.--(_R_) Cross-section of an organ pipe showing action of tongue at _C_. (_a_) The fundamental tone in a closed pipe has a wave length four times the length of the pipe; (_b_) and (_c_) how the first and second overtones are formed in a closed pipe; (_d_) the fundamental tone of an open pipe has a wave length equal to twice the length of the pipe; (_e_) and (_f_) first and second overtones of open pipe.] If an _open_ organ pipe be sounded by blowing gently through it, a tone of definite pitch is heard. Now if one end is closed, on being sounded again the pitch is found to be an octave lower. Therefore, _the pitch of a closed pipe is an octave lower than that of an open one of the same length_. =350. Nodes in Organ Pipes.=--Fig. 341, _R_ represents a cross-section of a wooden organ pipe. Air is blown through _A_, and strikes against a thin tongue of wood _C_. This starts the jet of air vibrating thus setting the column of air in vibration so that the sound is kept up as long as air is blown through _A_. To understand the mode of vibration of the air column a study of the curve that represents wave motion (Fig. 342) is helpful Let _AB_ represent such a curve, in this 2, 4 and 6 represent nodes or points of least vibration, while 1, 3 and 5 are antinodes or places of greatest motion. A full wave length extends from 1-5, or 2-6. Now in the open organ pipe (Fig. 341_d_), the end of the air column _d_ is a place of great vibration or is an antinode. At the other end also occurs another place of great vibration or an antinode; between these two antinodes must be a place of least vibration or a node. The open air column therefore extends from antinode to antinode (or from 1-3) or is _one-half_ a wave length. _The closed air column_ (Fig. 341_a_) extends from a place of _great_ vibration at _a_ to a place of _no_ vibration at the closed end. The distance from an antinode to a node is that from 1-2 on the curve and is _one-fourth_ a wave length. [Illustration: FIG. 342.--Graphic representation of sound waves.] [Illustration: FIG. 343.--A clarinet.] When a pipe is blown strongly it yields overtones. The _bugle_ is a musical instrument in which notes of different pitch are produced by differences in blowing. (See Fig. 341.) (_d_), (_e_), (_f_). In playing the _cornet_ different pitches are produced by differences in blowing, and by valves which change the length of the vibrating air column. (See Fig. 334.) The _clarinet_ has a mouthpiece containing a reed similar to that made by cutting a tongue on a straw or quill. The length of the vibrating air column in the clarinet is changed by opening holes in the sides of the tube. (See Fig. 343.)