One great difficulty with such a differentiation of the three types of rhythms presents itself when one inquires into the objective regularity of the types; the fact is that music is by far the most regular in its time values, though it has more content than the sound series; and that just as great irregularities are possible in the bare sound series as in the rhythm of verse with its rich and definite content.
Later statements of the facts and theories relating to rhythm have inclined more and more to an emphasis of the motor aspect, even on the part of Wundtians. Since Meumann there has been some detailed laboratory work published, but the amount of accurately measured rhythmic material is astonishingly small. Meumann established experimentally the well-known relation between the length of a rhythmic element and its accent, and corroborated the earlier work on subjective accentuation. The reports contain the measurements of but about eighty individual unit groups (iambs, trochees, etc.). Ebhardt gave the measurements of from 150 to 300 taps from each of three subjects. But his work is vitiated, as far as any application to rhythm is concerned, because he based everything on the judgment of equality, which has nothing to do with rhythm.
 Ebhardt, K.: Zeilschr.
f. Psych, u. Physiol. d.
Sinnesorgane,1898, Bd. 18, S. 99.
Hurst, McKay and Pringle published measurements of about 600 individual unit groups from three different subjects; in several cases, the material consists rather too much of records of the experimenters themselves, but in general their results agree very well with those of other authors. Scripture published the measurements of a single stanza of poetry. It is but a single stanza and quite too little material on which to base any conclusions, but it is notable as a measurement of freely spoken rhythm. No experiments have been published which bear on the nature of the rhythmic phrase, of the period, or of the stanza.
 Hurst, A.S., McKay, J., and
Pringle, G.C.F.: Univ. of
Toronto Studies, 1899, No. 3, p. 157.
 Scripture, E.W.: Studies
from the Yale Psych. Lab.,
1899, VII., p. 1.
Our problem is: What part do the recurrent qualitative factors, like rhyme, play in the grouping of rhythms? They function evidently, in the main, as factors determining the periods or larger phrases of the rhythm structure—the verses and stanzas of poetry and nonsense verse. As no work has been done on the nature of such larger rhythmic unities, a large share of the investigation was concerned with the nature of the verse unity.
Two methods of investigation were used: Subjects listened to rhythmic series, into which various modifications were introduced; and secondly, rhythms of a prescribed type, produced by the subject, were recorded and measured.
[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE IX. Opposite p. 417]