Upon p. 63 is given a picture of the form of a simple sound-wave in air, which, as described, consists of two parts, a condensation and a rarefaction. All simple sound-waves have such a form; but when two or more sound-waves that stand in some simple ratio to each other, as do the sounds of musical instruments, are formed in air, the resulting wave is more or less complex in structure; and where there are many components, as there are where a number of different kinds of instruments are all sounding at once, it is well-nigh impossible to figure even approximately the form of such wave-combinations. It is generally given in treatises upon sound with ordinates representing the factors with their relative intensities. When the extremities of the ordinates are connected, there is drawn a curved line with regularly recurring loops. This cannot give a correct idea of the form of the wave, because the motion of a particle of air is not up and down like a floating body upon waving water, but it is forward and back, in the direction of the motion of the wave. In Fig. 10 three simple sound-waves are thus represented at 1, 2, and 3, these having the wave-length 1, 2, and 3. In 4, the three are combined into one compound wave, and better show the form of a transverse section of such a sound-wave in the air. The organ-pipe called the principal gives out such a compound wave as is seen by referring to the table on p. 91. The second overtone, however, is quite weak in that pipe, which would so modify the form as to lessen somewhat the density at _b_, and increase it at _a_. [Illustration: FIG. 10.] In like manner the space in the length of the fundamental sound, whatever it may be, is divided up into a number of minor condensations and rarefactions, which may strengthen each other, or so interfere as to change the position of both; as is seen in the figure at _b_, where the condensation due to wave 2 interferes with the rarefaction of 3. CORRELATION. HAVING treated at some length of the three factors involved in telephony,--namely, electricity, magnetism, and sound,--it remains to follow up the various steps that have led to the actual transmission of musical sounds and speech over an ordinary electric circuit.