Diphthongs with ă as base.
If the short a is quickly followed by i and u, so that, as the Hindus say, the guttural is mixed with the palatal and labial vowels like milk and water, we get the diphthongs ai and au, pronounced as in French. They correspond in sound to the Italian e and o, and to the English sounds in sailor and home.
Diphthongs with ā as Base.
If the a, as the first element, retains more of its independent nature, or is long, then â + i pronounced together give the German diphthong ai, as in pie and buy; a + u give the German diphthong au, as in proud.
The formation of these sounds is well described in § 15. of the printed Rules: "Diphthongal sounds are formed by the combination of any two vowel sounds. They may be best ascertained and tested by first pronouncing each vowel distinctly and separately, and then making them gradually coalesce.
"Thus a-i, (English) ah-ee, ai, which forms the sound of the English i in ride, mile.
"A-u, au, forms the sound of the English ow in now.
"O-i, oi, as in voice."
Diphthongs with ĕ as Base.
If, instead of the short or long a, the base of the diphthong becomes ĕ, we get the combinations ei and eu, both of rare occurrence except in German, where the sound of ei (English isle), is thinner than that of ai (English ire). In eu, the two vowels are still heard very distinctly in the Italian Europa. In German they coalesce more, and almost take the sound of oy in boy.
Diphthongs with ŏ as Base.
In the diphthong oi also, the pronunciation may vary according to the degree of speed with which the i follows the ŏ. O and u, on the contrary, coalesce easily, and form the well-known deep sound of ou in bought, or of a in fall.
