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VIII. § 90.] COMBINATION-TONES. 179
In order to meet this objection, it will be necessary first to acquaint the reader with certain known experimental facts which Helmholtz has dragged out of the obscurity in which they had lain for fully a century, and forced to deliver their testimony in completion of his theory.
90. Let two tuning-forks of different pitch mounted on their respective resonance-boxes be thrown into powerful vibration by a resined bow. With adequate attention it is possible to recognise, in addition to the tones of the forks themselves, certain new sounds which usually differ in pitch from both the former ones. These sounds, called from the manner of their production combination - tones, fall into two categories, with only one of which, discovered in 1740 by a German organist named Sorge, we need now concern ourselves. It consists of a series classed as combination-tones of the first, second, third, &c, orders, of which the first is of the most importance, as it can be heard without difficulty. Its pitch is determined by the following law. The combination-tone of the first order of two simple primary-tones has for its vibration-number the difference between the respective vibration-numbers of the primaries. Thus, e.g., if the two primaries make 200 and 300 vibrations per second, and therefore form a Fifth with each other, the combination-tone will