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IX. § 96.] ADDITION OF INTERVALS. 189
by the first and third notes, i. e. of the sum of a Fifth and a Major Third ? We will, for shortness, call the three notes (1), (2), (3), in order of ascending pitch.
Our result, then, is the two vibration-numbers multiplied together. The reasoning is perfectly general, and gives us the following rule.
To find the vibration-fraction of the sum of two intervals, multiply their separate vibration-fractions together.
96. Next, take the opposite case. Let (2) be a Major Third above (1), and (3) a Fifth above (l), and let the vibration-fraction for the interval (2)—(3) be required.