The following are the five (5) Diphthongs which complete the Vowel Scale:
The IU is composed of the first Vowel I (ee) and the last U (oo). The I-sound, so placed before another Vowel-Sound, tends readily to be converted into or more properly to prefix to itself the weak Consonant-Sound represented in English by Y (in German and Italian by J); thus YIU for IU. The whole of the three Sounds so involved (a real Triphthong) are represented by the English U long—which is never a simple Vowel-Sound—as in union, pronounced yioonyun.
This Diphthong IU (or yiu) denotes Conjunction, Conjuncture, Event (the two ends meeting); and also Coupling or Unition; a central point between extremes.
The next and the most important of the Diphthongs (except AU) is AI, compounded of the third (A) and the first (I) of the simple Vowel-Sounds. It is pronounced very nearly like the English long I, as in pine, fine, etc., which is not a simple Vowel; but is compounded of the two simple Vowels above mentioned (A and I, ahee) in a very close union with each other; or, as it were, squeezed into each other. The Tikiwa (Tee-kee-wah) combination (this is the name of the Scientific Universal Language), AI, is not ordinarily quite so close, and when pronounced long, is quite open, so that each Vowel is distinctly heard (ah-ee).
This Diphthong AI may be regarded as embracing and epitomizing the lower or ground wing or half of the Simple Vowel-Scale (I E a A); its meaning is, therefore, that of Basic or Substantial Reality: the Ground of Existence.
Contrasted with this is the next Diphthong, OI (aw-ee), compounded of the fifth (o) and the first (I) Vowel-Sounds. It is the Sound of oy in boy. The I contained in this Diphthong may be regarded as standing in the place of U at the other extremity of the Scale. This last Sound has a tendency to return into I through the French slender U, illustrating the Principle of the Contact of Extremes. The Diphthong OI may, therefore, be viewed as embracing and epitomizing the upper or ethereal wing or half of the Simple Vowel Scale (o u O U); its meaning is, therefore, that of Aerial or Ascending Reality; Loftiness or Loft.
Next there occurs a Diphthong OI, pronounced as the same letters in the English word going, which has a half claim to be ranked with the Leading Diphthongs. It is sometimes reckoned into, and sometimes out of, the Scale—like a among the Simple Vowels. Its meaning is that of Frontness, Prospect.
Finally, the great Focal Diphthong, that which includes and epitomizes the whole Vowel Scale, is AU (ah-oo), compounded of the third Vowel-Sound (A) and the Seventh (or Eighth) U. It is the sound heard in our, or in the Spanish causa. The meaning of this Supreme Diphthong and general Vowel Representative is Universal Reality. It stands practically in the place of all the Vowels, in the Composition of Words of an inclusive meaning. That is to say, it integrates in its signification, all that is inherently signified by all the other Vowels.
While, however, AU is practically and usually the Representative, Analogue or Equivalent, in the Domain of Language, of Universal Reality among the Elements of Being, this is so only in practice. Theoretically, the Diphthong best adapted to represent this Idea is AO; the A and the O being, in a supreme sense, the two most prominent or leading Vowels. But it is a little difficult to retain the Organs of Utterance in the position which they must assume in order to pronounce these two Vowel-Sounds in conjunction. The organs readily and naturally slide into the easier position in which they utter AU. This is correspondential with the difficulty always experienced in adhering to Pure Theory (O); and the natural tendency to glide from it, as ground too high for permanent occupation, into the more accommodating Domain of the Practical (U).
The Full Scale of Vowel Sounds coupled with the Full Scale of the (Indeterminate) Realities of Universal Being is, therefore, as follows: