In the construction of any complicated piece of work, be it a mansion or even a musical instrument, which we wish to have well finished, the first thing naturally sought of it is good materials; the idea may be justly applied to the formation of the Voice.

The material required is a pure tone, by a pure tone is meant that, which is entirely free from any nasal, guttural, or dental sound; the best mode of acquiring this pure tone, is by vocalizing on the vowel A, as pronounced by the Italians, or the Scotch. Ah. It is been found from experience, that by emitting the sound on the vowel detached, it is possible that the tongue may by some trifling motion injure the quality of tone, in passing through the mouth, consequently it is deemed necessary to add the consonant L, making La; it will be observed that by this addition the tongue, in uttering the syllable, will touch the roof of the mouth and gently fall into the cavity in the bottom of it; the top of the tongue, touching the inner part of the lower gums.

The teeth should be kept sufficiently apart to admit not more than the forefinger, for if they are widely separated, or even admit two fingers, an inclination to a nasal tone is created, and on the other hand, if they are too close a dental tone will arise. In the conservatories in Italy, there are men employed expressly to remind the pupils of the necessity of keeping the mouth sufficiently open, by constantly calling out "Aprite le bocca" open the mouth.

The lower lip should be inclined to the smile, so that the tip of the under teeth may just be seen; for if the lip is allowed to remain in its natural position during practice, the probability is, that it will weaken and damp the tone. The greatest care should be taken to avoid any distortion of the features, such as the movement of the eyes or eyebrows; as this cannot render the least assistance in the production of a sweeter, a larger, or a better quality of tone, the Voice coming as it does (be it "Voce di petto" the Voice of the chest or "Voce di testa" the Voice of the head), from the back of the mouth, the head should be kept in a horizontal position, the practitioner avoiding the slightest movement of the lower jaw, otherwise he may cause a tightness of the muscular action, (a sensation so frequently felt by those who have not studied the art of singing) while passing from the chest to the head.

—William Huckel, Practical Instructions for the Cultivation of the Voice; With a Series of Rules for its Adaption to the Chamber, the Concert Room, and the Stage (1820?): page 18-20.

Huckel was a student of Domenico Corri, himself a student of Nicola Porpora. His text is of great interest, not only for its connection to Porpora—whose reputation as a voice teacher is legendary—but as the title indicates, for its practical instruction. He offers the reader much to ponder, one key point in the passage above being "the lower lip should be inclined to the smile..."

Find Huckel's text on the download page.