2. No more than one simple sound can be formed by one aperture of the mouth, and one position of the organs of speech. The only difference that can be made with the same position of the organs, is, to prolong and shorten the same sound.
3. Two simple sounds, closely united in pronunciation, or following each other so rapidly that the distinction is scarcely perceptible, form a dipthong. In pronouncing a dipthong, two positions of the parts of the mouth are required.
4. Those letters which are not marks of articulate sounds, but represent indistinct sounds, formed by some contact of the parts of the mouth, or by compressing those parts, check all sound, are denominated consonants.
By the first definition we ascertain the number of vowels in English. In pronouncing each of the letters a1, a4, a3, e1, o1, o6, u2, we observe but one position or aperture of the mouth; the sounds are therefore simple, and the letters are called vowels. The six first sounds are capable of being prolonged at pleasure.
By the second definition, we determine which sounds are the same in quality, and different only in the time of being pronounced. Thus i in fit has the same quality of sound as ee in feet, for both are pronounced with the same disposition of the organs; but the first is the shortest articulation of the sound, and the last, a long or grave articulation. The other vowels have also their short or abrupt sounds; a in late has its short sound in let; a in cart has its short sound in carry; a in fall has its short sound in folly; oo in fool its short sound in full. O is sometimes shortened in common parlance, as in colt; but the distinction between o in coal and colt, seems to be accidental or caused by the final consonant, and not sufficiently settled or important to require a separate consideration.
By the third definition we are enabled to ascertain the dipthongs in our language. The letters i, u and y are usually classed among the vowels; but the first or long sound of each requires, in pronunciation, two positions of the organs of speech, or rather a transition from the position necessary to form one simple sound, to the position necessary to form another simple sound. We begin the sound of i nearly with the same aperture of the glottis, as we do the broad a or aw: The aperture however is not quite so great: We rapidly close the mouth to the position where we pronounce ee, and there stop the sound. This letter is therefore a dipthong. Y has no property but what belongs to i.
U also is not strictly a vowel; nor is it, as it is commonly represented, composed of e and oo. We do not begin the sound in the position necessary to sound ee, as is obvious in the words salute, salubrious, revolution; but with a greater aperture of the mouth and with a position perfectly easy and natural. From that position we pass to the position with which we pronounce oo, and there close the sound.
It must however be observed that when these letters, i, u, are followed by a consonant, the two sounds of the dipthong are not clearly distinguishable. We do not, in fight, hear the sound of ee; nor the sound of oo in cube. The consonant compresses the organs and closes the sound of the word so suddenly, that the ear can distinguish but a simple vocal sound: And notwithstanding these letters are dipthongs, when considered by themselves, yet in combination with consonants, they are often marks of simple sounds or vowels.
The short sound of i and y, is merely short ee. The sound of u in tun, is a separate vowel, which has no affinity to any other sound in the language.[H]
The sound of oi or oy is dipthongal, composed of the third or broad a, and ee. The sound of ou or ow is also dipthongal, compounded of third a and oo. The sound however does not require quite so great an aperture of the mouth as broad a; the position is more natural, and the articulation requires less exertion.