Accent in Gr. (προσῳδία) is explained by Dion. Hal. περὶ συνθέσεως ὀνοµάτων ch. xi. as a distinct difference of musical pitch in pronouncing the syllables of a word, those having the grave or heavy accent (βαρεῖαgravis) being spoken at a comparatively low pitch, those having the acute or sharp accent (ὀξεῖαacūtus) being spoken as nearly as possible a musical Fifth higher (διὰ πέντε), and those having the circumflexaccent (περισπωµένηcircumflexus) beginning in the high pitch and descending a Fifth during the pronunciation of the same syllable. The same three varieties occurred in Latin, but their positions in a word followed very different laws. This variety of pitch disappeared for both Latin and Greek towards the end of the Third Century a.d. when the feeling of quantity was lost, and the high pitch in Greek and Latin became merely greater force, and this stress accent has remained the substitute for musical accent in modern Greek, in Italian and Spanish, and is also found in German and English. In Swedish and Norwegian a musical syllabic accent remains in use; in Danish it is replaced in some circumstances by a peculiar catch, and in others by stress, as in English. In French, where probably stress was at one time strongly marked, the difference for at least three centuries has been so slight that writers have disputed as to its nature and the position of the stress syllable. In all languages having the stress, a variable alteration of pitch and quality of tone always prevails, and is used to express varieties of feeling. This expression belongs to rhetoric. The grammatical varieties of accent in English are great, but are all varieties of stress. The position is fixed in words of more than one syllable. Monosyllables have various degrees of stress according to circumstances. Hence the distinction of syllabicaccent for the first, and verbalaccent, phraseaccent, or emphasis for the second.
ibidem, sense 2.a:
The old Latin forms (´) acūtus, (ˋ) gravis, (ˆ) circumflexus, are retained, but each one now represents mere stress, except in works on elocution where (´) now generally represents a rising (not a fixed high) pitch; (ˋ) a falling pitch (the ancient circumflex), and (ˆ) a rising followed by a falling pitch, not used in ancient Latin and Greek. Some writers use (ˆ) for length only, the same as (ˉ). The old meanings are quite lost.
ibidem, sense 2.b:
The old ´ ˋ ˆ are mostly used, as French eéèê in je, été, tiède, même, but a great variety of other signs have also been introduced. These diacritical accents sometimes distinguish meaning only, as French aà, lalà. These marks are not used in English orthography. But sometimes ˋ is used to shew that -ed is to be pronounced as a distinct syllable, as learnèd, hallowèd, and some write é for a final e pronounced, as Hallé (properly German Halle).
ibidem, sense 3:
This utterance consists mainly in a prevailing quality of tone, or in a peculiar alteration of pitch, but may include mispronunciation of vowels or consonants, misplacing of stress, and misinflection of a sentence. The locality of a speaker is generally clearly marked by this kind of accent.
ibidem, sense 6 (page 51/2):
English verse is theoretically marked by a periodical recurrence of strong syllables, having a loud stress, a certain number of times in a line, separated by one or two weak or unaccented syllables. The habits of poets do not however carry out this theoretical law. Thus in ‘to err is human, to forgive divine,’ theory would require to to be strong; similarly in ‘for the poor craven bridegroomsaid nevera word,’ theory would require the first syllable in craven to be weak and both groom and said to be as weak as the -ver and a which follow. They are not so. Hence has arisen the conception of rhythmically or metrically accented and unaccented syllables, as distinguished from the grammatically or verbally accented syllables. Thus, in the above examples, to has the rhythmical and not the verbal or phrase accent, and craven has the syllabic but not the rhythmical accent; err has both verbal and rhythmical accent, divine has both syllabic and rhythmical accent.
