The foregoing examples, although following the pulse in their exactness, are still for scientific purposes not quite what may be desired. The heart's action varies. So do musical tempi. Both are disturbed by the slightest exciting or nervous influences. Still the track, though faint at times, sometimes quite effaced by conscious effort, is there; corroborated through a hundred different channels. One distinguished psychologist[1] finds that a subject could repeat simple intervals without accent with greatest exactness when these intervals lay between 0.4 and 0.7 seconds. It takes but a simple problem in arithmetic to see that this agrees with from 75 to 86 rhythmic beats per minute, or the region of pulsation common to the human pulse. Another[2] on conducting a series of experiments on rhythm, 'the first and most important object of which was to determine what the mind did with a series of simple auditory impressions in which there was absolutely no change of intensity, pitch, quality or tone interval,' finds that the pulse seemed at times to impose a grouping in which the clicks coming nearest to the time of the heart beats were accented.
To Professor Bolton[3] must be given the credit of having successfully found the means by which rhythm can be permanently differentiated from time in music. He says this general principle, arrived at by the same experiments, may be stated: "The conception of a rhythm demands a perfectly regular sequence of impressions within the limits of one second and one hundredth of a second. When a longer interval was introduced into the series, the impressions coming between the long intervals fell together into a group but they did not form an organic unity. There was no pleasure in such a rhythm. Something seemed to be looked for in this longer interval which was wanting." Why?
No matter how slowly one sound follows another, time, as understood in music, can still be a characteristic of the sequence. A clock may strike this minute and not again for an hour, but time is still being measured. A rhythm, however, can be said to exist only when sounds succeed each other so as to fall within the same limited horizon of attention. This differentiation has not to this day been clearly made by authors of musical encyclopedias and dictionaries, they having been satisfied with considering rhythm as simply similar in music to meter in verse.
Bearing these statements in mind, it seems improbable that the mere physical activities and industries of primitive peoples, such as cradle-rocking, spinning and grinding should have been so constantly of one rhythm as to impress accidentally a beat of such uniform variation, extending within fifteen pulsations difference a minute
