To conclude in the words of-Peter MacCarthy (English Pronunciation) "A pure vowel is one for which the organs of speech remain in a given position for an appreciable period of time. A diphthong is a vowel sound consisting of a deliberate, ie. intentional glide, the organs of speech starting in the position of one vowel and immediately moving in the direction of another vowel. A diphthong, moreover, consists of a single syllabic -that is, the vowel-glide most be performed with a single impulse of the breath; if there is more than one impulse of breath, the ear perceives two separate syllables..."
INTRODUCTION TO LINGUISTICS & PHONETICS - CLASSIFICATION AND DESCRIPTION OF SPEECH SOUNDS: ENGLISH VOWELS