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. The union of _a_ and _w_ in _law_, has been very erroneously considered a dipthong. Whatever might have been the ancient pronunciation of these letters (and it is probable that good reasons operated to produce their union) they now exhibit but one simple vocal sound. The same may be observed of _ee_, _oo_, _au_, _ai_, _ea_, _ei_, _ie_, _eo_, _oa_, and perhaps some other combinations, each of which actually exhibits the sound of one letter only, which sound is as simple as that of _a_ or _o_.[38]