In semivocalibus similiter sunt praepositivae aliis semivocalibus in eadem syllaba; ut m sequente n, ut Mnesteus, amnis.
Each letter has its ‘time,’ or ‘times.’ Thus a short vowel has the time of one beat (mora); a long vowel, of two beats; a single consonant, of a half beat; a double consonant, of one beat. Theoretically, therefore, a syllable may have as many as three, or even four, tempora; but practically only two are recognized. All over two are disregarded and each syllable is simply counted ‘short’ (one beat) or ‘long’ (two beats).
Priscian says:
[Keil. v. II. p. 52.] In longis natura vel positione duo sunt tempora, ut do, ars; duo semis, quando post vocalem natura longam una sequitur consonans, ut sol; tria, quando post vocalem natura longam duae consonantes sequuntur, vel una duplex, ut mons, rex. Tamen in metro necesse est unamquamque syllabam vel unius vel duorum accipi temporum.
[ Accent.]
The grammarians tell us that every syllable has three dimensions, length, breadth and height, or tenor, spiritus, tempus:
[Keil. Supp. p. XVIII.] Habet etiam unaquaeque syllaba altitudinem, latitudinem et longitudinem; altitudinem in tenore; crassitudinem vel latitudinem, in spiritu; longitudinem in tempore.
Diomedes says:
[Keil. v. I. p. 430.] Accentus est dictus ab accinendo, quod sit quasi quidam cujusque syllabae cantus.
And Cicero: