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VIII. § 90.] INTENSITY OF COMBINATION-TONES. 181
shown in the treble stave are played on that instrument while the pressure of air in the bellows is vigorously sustained, the corresponding combination-tones of the first order, written in the bass, come out with unmistakable clearness. With pianoforte-sounds, on the contrary, combination-tones can be recognised only when the primaries are struck very forcibly, and they are always extremely faint and rapidly evanescent. If, however, the key corresponding to the combination-tone sought be first silently pressed down, its note is sometimes sustained by resonance when the keys of the two primaries have been smartly struck 'staccato'1.
Combination-tones of the second order may be treated as if they were first-order tones produced between one or other of the primaries and the combination-tone of the first order. Similarly we may regard each combination-tone of the third order as due to a second-order tone paired either with one of the primaries, with the first-order tone or with its own fellow of the second order. Successive subtraction will therefore enable us to determine the vibration-number of a combination-tone of any order from the vibration-numbers of the two primaries.
Combination-tones grow rapidly feebler as their
1 i.e. allowed to rise again as quickly as possible after the blow.