Early in the 18th century, Pope affirmed in his Essay on Criticism (1711) the classic doctrine of imitation. Prosody was to be more nearly onomatopoetic; the movement of sound and metre should represent the actions they carry:

’Tis not enough no harshness gives offence,
The sound must seem an Echo to the sense:
Soft is the strain when Zephyr gently blows,
And the smooth stream in smoother numbers flows;
But when loud surges lash the sounding shoar,
The hoarse, rough verse should like the torrent roar.
When Ajax strives some rock’s vast weight to throw,
The line too labours, and the words move slow;
Not so, when swift Camilla scours the plain,
Flies o’er th’ unbending corn, and skims along the main.

In 18th-century theory the doctrine of imitation was joined to numerous strictures on “smoothness,” or metrical regularity. Theorists advocated a rigid regularity; minor poets composed in a strictly regular syllable-stress verse devoid of expressive variations. This regularity itself expressed the rationalism of the period. The prevailing dogmas on regularity made it impossible for Samuel Johnson to hear the beauties of Milton’s versification; he characterized the metrically subtle lines of “Lycidas” as “harsh” and without concern for “numbers.” Certain crosscurrents of metrical opinion in the 18th century, however, moved toward new theoretical stances. Joshua Steele’s Prosodia Rationalis (1779) is an early attempt to scan English verse by means of musical notation. (A later attempt was made by the American poet Sidney Lanier in his Science of English Verse, 1880.) Steele’s method is highly personal, depending on an idiosyncratic assigning of such musical qualities as pitch and duration to syllabic values; but he recognized that a prosodic theory must take into account not merely metre but “all properties or accidents belonging to language.” His work foreshadows the current concerns of the structural linguists who attempt an analysis of the entire range of acoustic elements contributing to prosodic effect. Steele is also the first “timer” among metrists; that is, he bases his scansions on musical pulse and claims that English verse moves in either common or triple time. Twentieth-century critics of musical scanners pointed out that musical scansion constitutes a performance, not an analysis of the metre, that it allows arbitrary readings, and that it levels out distinctions between poets and schools of poetry.

The 19th century

With the Romantic movement and its revolutionary shift in literary sensibility, prosodic theory became deeply influenced by early 19th-century speculation on the nature of imagination, on poetry as expression—“the spontaneous overflow of powerful feelings,” in Wordsworth’s famous phrase—and on the concept of the poem as organic form. The discussion between Wordsworth and Coleridge on the nature and function of metre illuminates the crucial transition from Neoclassical to modern theories. Wordsworth (in his “Preface” to the Lyrical Ballads, 1800) followed 18th-century theory and saw metre as “superadded” to poetry; its function is more nearly ornamental, a grace of style and not an essential quality. Coleridge saw metre as being organic; it functions together with all of the other parts of a poem and is not merely an echo to the sense or an artifice of style. Coleridge also examined the psychological effects of metre, the way it sets up patterns of expectation that are either fulfilled or disappointed:

As far as metre acts in and for itself, it tends to increase the vivacity and susceptibility both of the general feelings and of the attention. This effect it produces by the continued excitement of surprize, and by the quick reciprocations of curiosity still gratified and still re-excited, which are too slight indeed to be at any one moment objects of distinct consciousness, yet become considerable in their aggregate influence. As a medicated atmosphere, or as wine during animated conversation; they act powerfully, though themselves unnoticed. Where, therefore, correspondent food and appropriate matter are not provided for the attention and feelings thus roused, there must needs be a disappointment felt; like that of leaping in the dark from the last step of a staircase, when we had prepared our muscles for a leap of three or four.

Biographia Literaria, XVIII (1817)

Romantic literary theory, although vastly influential in poetic practice, had little to say about actual metrical structure. Coleridge described the subtle relationships between metre and meaning and the effects of metre on the reader’s unconscious mind; he devoted little attention to metrical analysis. Two developments in 19th-century poetic techniques, however, had greater impact than any prosodic theory formulated during the period. Walt Whitman’s nonmetrical prosody and Gerard Manley Hopkins’s far-ranging metrical experiments mounted an assault on the traditional syllable-stress metric. Both Whitman and Hopkins were at first bitterly denounced, but, as is often the case, the heresies of a previous age become the orthodoxies of the next. Hopkins’s “sprung rhythm”—a rhythm imitating natural speech, using mixed types of feet and counterpointed verse—emerged as viable techniques in the poetry of Dylan Thomas and W.H. Auden. It is virtually impossible to assess Whitman’s influence on the various prosodies of modern poetry. Such American poets as Hart Crane, William Carlos Williams, and Theodore Roethke all used Whitman’s long line, extended rhythms, and “shaped” strophes.