How do you scan line 621 of De Rerum Natura, book VI, shown in bold below?
Praeterea, magnam sol partem detrahit æstu.
Quippe videmus enim vestes humore madentes
Exsiccare suis radiis ardentibu’ solem.
At pelage multa et late substrata videmus.
Proinde, licet quamvis ex unoquoque loco sol
Humoris parvam delibet ab æquore partem;
Largiter in tanto spatio tamen auferet undis.
(My amateur translation: "Furthermore, the Sun draws off a large part of [the oceans' water] with its heat. Certainly we see that the Sun dries out clothes dripping with moisture with its fiery rays. And we see the many seas stretched out below [the Sun]. So, however small a part of the moisture the Sun takes from the sea out of any one spot, it still takes abundantly in such a space from the waves." I'm not sure of this translation; I've asked a question about it here.)
If I assume that Lucretius is lengthening the first syllable of proinde, I get:
Prōin/dē licet / quamvīs / ex ū/nōquoque / locō sōl
That last foot is crazy. It doesn't have anything like the cadence you normally get in hexameter. Is this just a flaw in Lucretius's verse, am I mis-scanning it, or is there some hexameter custom at work here that I'm not aware of?