Taking it then for granted, that verse makes upon the ear a different impression from that of prose; nothing remains but to explain this difference, and to assign its cause. To these ends, I must call to my aid an observation made above in treating of the sound of words, that they are more agreeable to the ear when composed of long and short syllables than when all the syllables are of the same sort. A continued sound in the same tone, makes an impression that comes not up to any idea we have of music. The same note successively renewed by intervals, is more agreeable; but still makes not a musical impression. To produce this impression, variety is necessary as well as number. The successive sounds or syllables, must be some of them long, some of them short; and if also high and low, the music is the more perfect. Now if this impression can be made by single words, much more by a plurality in an orderly succession. The musical impression made by a period consisting of long and short syllables arranged in a certain order, is what the Greeks call _rhythmus_, the Latins, _numerus_, and we _modulation_ or _measure_. Cicero justly observes, that in one continued sound there is no modulation: “Numerus in continuatione nullus est.” But in what follows he is wide of the truth, if by _numerus_ he means modulation or musical measure. “Distinctio, et æqualium et sæpe variorum intervallorum percussio, numerum conficit; quem in cadentibus guttis, quod intervallis distinguuntur, notare possumus.” Falling drops, whether with equal or unequal intervals, are certainly not musical. We begin then only to be sensible of a musical expression, when the notes are varied. And this also was probably the opinion of the author cited, though his expression be a little unguarded[97].
It will probably occur, that modulation, so far as connected with long and short syllables combined in a sentence, may be found in prose as well as in verse; considering especially, that in both, particular words are accented or pronounced in a higher tone than ordinary; and therefore that the difference betwixt them cannot consist in modulation merely. The observation is just; and it follows, that the distinction betwixt prose and verse, since it depends not on modulation merely, must arise from the difference of the modulation. This is precisely the case, though the difference cannot with any accuracy be explained in words. Verse is more musical than prose; and of the former, the modulation is more perfect than of the latter. The difference betwixt verse and prose, resembles the difference in music properly so called betwixt the song and the recitative. And the resemblance is not the less complete, that these differences, like the shades of colours, approximate sometimes so nearly as scarce to be discernible. A recitative in its movement approaches sometimes to the liveliness of a song; which on the other hand degenerates sometimes toward a plain recitative. Nothing is more distinguishable from prose, than the bulk of Virgil’s hexameters. Many of those composed by Horace, are very little removed from prose. Sapphic verse has a very sensible modulation. That on the other hand of an Iambic, is extremely faint[98].