[641] Ristoro also defines apogee and perigee (“auge” and “opposito d’auge”) as effects of movements on an epicycle, and says that the deferent is “declined” from the ecliptic (“declinato della via del sole,”) part being north and part south, but he does not say whether its perigee is north or south. Comp. del Mondo. I. xii. xiv., and III. vii.
[642] For a thorough discussion of this question see Moore’s Studies in Dante ii. pp. 303-374.
[643] “The foundation of your elements.” Par. xxix. 51.
[644] “Rises highest towards heaven out of the sea.” Purg. iii. 15.
[645] “Dry vapour.”
[646] “Exhalations from the water.”
[647] “Behind [i.e. following] the heat.”
[648] Purg. xxviii. 97-99 and xxi. 52-53. The gate is meant by “i tre gradi” (“the three steps”), and “ove si serra” (“where it is locked”). See Purg. ix. 76, 106-108.
“ ... The universal atmosphere Turns in a circuit with the primal motion, Unless the circle is broken on some side: Upon this height, that all is disengaged In living ether, doth this motion strike, And make the forest sound, for it is dense.” (Longfellow).