Glides are only transitional sounds. They are intermediate to Consonants and Vowels, combining the characteristics of certain central-aperature consonants with the wide or expanded quality of vowels, but differing from vowels in not having a fixed configuration. ...in representing glides the peculiarities of consonants and vowels are blended: the accented fingers, by being straightened, contribute a consonant characteristic; while the second phalanx of the thumb, by being held at an angle to the plane of the palm, imparts to the glide positions the wide, without giving them the firm, quality of vowel positions.
Lyon, Edmund The Lyon Phonetic Manual (Rochester, NY: Deaf-Mute Institution, 1891)
Courtesy the private collection of Roy Winkelman