Noah Webster
Diph-thong
DIPH-THONG, noun [Gr., sound; Latin ] A coalition or union of two vowels pronounced in one syllable. In uttering a diphthong, both vowels are pronounced; the sound is not simple, but the two sounds are so blended as to be considered as forming one syllable, as in joy, noise, bound, out. [The pronunciation dipthong is vulgar.]