142 HARMONIC SOUNDS NODES. SECT. XVII. 



the ear also, vibrates in unison with each at the same instant 

 (N. 181). 



Harmony consists in an agreeable combination of sounds. 

 When two chords perform their vibrations in the same time, 

 they are in unison ; but, when their vibrations are so related as 

 to have a common period, after a few oscillations they produce 

 concord. Thus, when the vibrations of two strings bear a very 

 simple relation to each other, as where one of them makes two, 

 three, four, &c., vibrations in the time the other makes one ; or, 

 if it accomplishes three, four, &c., vibrations while the other 

 makes two, the result is a concord which is the more perfect the 

 shorter the common period. In discords, on the contrary, the 

 beats are distinctly audible, which produces a disagreeable and 

 harsh effect, because the vibrations do not bear a simple relation 

 to one another, as where one of two strings makes eight vibra- 

 tions while the other accomplishes fifteen. The pleasure afforded 

 by harmony is attributed by Dr. Young to the love of order, and 

 to a predilection for a regular repetition of sensations natural to 

 the human mind, which is gratified by the perfect regularity 

 and rapid recurrence of the vibrations. The love of poetry and 

 dancing he conceives to arise in some degree from the rhythm of 

 the one and the regularity of the motions in the other. 



A blast of air passing over the open end of a tube, as over the 

 reeds in Pan's pipes ; over a hole in one side, as in the flute ; or 

 through the aperture called a reed, with a flexible tongue, as in 

 the clarinet, puts the internal column of air into longitudinal 

 vibrations by the alternate condensations and rarefactions of its 

 particles. At the same time the column spontaneously divides 

 itself into nodes, between which the air also vibrates longitudi- 

 nally, but with a rapidity inversely proportional to the length of 

 the divisions, giving the fundamental note or one of its har- 

 monics. The nodes are produced on the principle of interferences 

 by the reflection of the longitudinal undulations of the air at the 

 ends of the pipe, as in the musical string, only that in one case 

 the undulations are longitudinal, and in the other transverse. 



A pipe, either open or shut at both ends, when sounded, 

 vibrates entire, or divides itself spontaneously into two, three, 

 four, &c., segments separated by nodes. The whole column 

 gives the fundamental note by waves or vibrations of the same 

 length with the pipe. The first harmonic is produced by waves 



