47. Two unlike (43-46) vowels pronounced under one stress and as one
syllable form a _Diphthong_. All diphthongs are long.
In all diphthongs the transition from one vowel to the other is gradual.
A diphthong is, therefore, not formed simply by pronouncing two vowels
in succession, but the vocal organs pass through all the intermediate
positions and consequently the sound is constantly changing.
48. In their origin diphthongs are of two kinds: (_a._) primitive
diphthongs: as in #foedus#, _treaty_; #aurum#, _gold_; or (_b._)
secondary diphthongs, the result of vowels meeting in formation,
composition, or inflection: see 120.
49. The diphthongs which occur in classical Latin are #au#, #ae#, #oe#,
and the rare #ui# and #eu#.
#au# sounded like _ou_ in _house_. #ae# had the sound of short Latin #a#
rapidly combined with the sound of _e_ in English _men_. But it is the
common practice now to give to #ae# the sound of _ay_ or _ai_ in _ay_,
_aisle_, although the difference between Latin #ae# and the earlier #ai#
from which it descended is thus obliterated. #oe# had the sound of short
Latin #o# rapidly followed by the sound of _e_ in English _men_. But it
is now customary not to distinguish between Latin #oe# and #oi#, and to
give to both the sound of _oi_ in _boil_. #ui# is pronounced by
combining Latin short #u# and #i# (40, 41) with the stress on the #i#
like French _oui_; #eu# by combining Latin short #e# and #u# with stress
on the #u#.
50. Besides these, the following diphthongs occur in the older
inscriptions: #ai# pronounced as _ai_ in _aisle_; #ei# as _ei_ in
_eight_; #oi# as _oi_ in _boil_; and #ou# which sounded very much like
the final _o_ in _no_, _go_, which is really a diphthong (see 39).