In Horace such a final syllable is never lengthened before a word beginning with two consonants.
[Hidden Quantity.]
[2459.] A vowel which stands before two consonants, or a double consonant, belonging to the same word, so that its natural quantity cannot be determined from the scansion of the word, is said to possess Hidden Quantity.
[2460.] The natural quantity of such a vowel may sometimes be ascertained: (a.) from the statements of ancient writers; (b.) from the way in which the vowel is written in Latin inscriptions (see [24], 29); (c.) from the transliteration of the word into other languages, especially Greek; (d.) from the etymology of the word, or from a comparison of it with kindred words in other Indo-European languages; (e.) from comparison with derived words in the Romance languages. But all these kinds of evidence must be used with great caution.
[2461.] For the length of a vowel before ns, nf, and certain other groups of consonants, see [122].
[2462.] In inceptive verbs ([834]) the ending -scō is thought to be always preceded by a long vowel: as, crēscō, nāscor, proficīscor.
[2463.] In the perfect indicative active, perfect participle passive and kindred formations of verbs in -gō preceded by a short vowel, as agō, regō, the theme syllable shows a long vowel: as, lēxī, rēxī, tēxī; āctus, lēctus; rēctor; āctitō.
[(B.) Some Peculiarities of Quantity in Old Latin.]
[2464.] For the preservation of a long vowel in certain specific endings in old Latin, see [132].
[2465.] Final -āl is sometimes preserved long in the nominative singular: as, bacchānāl (Plaut.); also the syllable -es in the nominative singular of stems in -t- which have the genitive in -itis ([477]): as mīlēs (Plaut.) 171, 1.