It is a vowel in the articulation of which there is no weakening in the force of articulation towards the end.

...are those in the articulation of which the bulk of the tongue is in the most forward position in the mouth cavity; the resonator is in the back part of the mouth cavity; the front part of the tongue is raised in the direction of the hard palate.

It is a vowel in the production of which the organs of speech change their position but very slightly.

It a pure vowel in the articulation of which the organs of speech do not change their position throughout the duration of the vowel.

... are those in the articulation of which the raised part of the tongue is very low in the mouth cavity. The air-passage is very wide.

... are those in the production of which the tongue is neither advanced nor retracted; the resonator is equally open along the mouth cavity; the central part of the tongue is raised towards the juncture between the hard and the soft palate

... are those in the articulation of which the raised part of the tongue is at the medium position in the mouth cavity. 

... are those in the articulation of which the bulk of the tongue is in the most backward position; the resonator is in the front part of the mouth cavity; the bulk of the tongue is raised in the direction of the soft palate.

... are those in the articulation of which the raised part of the tongue is in the highest position in the mouth cavity. The air-passage is rather narrow. 

It is a complex vowel which consists of two elements forming one syllable.

... are those in the articulation of which the bulk of the tongue is in the back part of the mouth cavity but less back than for back vowels; the resonator is in the front part of the mouth cavity; the back part of the tongue is raised in the direction of the front of the soft palate