From “A Few Words of Advice to Singers” in an 1813 songbook called “David’s Harp”:
Let the mouth be opened freely, but not wide, and let the tones proceed from the chest — otherwise they cannot be good.
Avoid singing as though the nose was stopped up; — this is commonly called “singing through the nose,” but it is the very reverse of it, as may be proved by closing the nostrils.
Never attempt to sing a part for which your voice is not calculated; for if you strive to reach tones which are above your compass — your abortive attempt will have a tendency to depress the pitch of the tune and create unpleasant sensations in yourself and others — men who cannot reach F with ease, had better sing Bass.
Stand or sit erect, and avoid all ridiculous gesticulation and affectation; “suit your looks and action to the words,” and if the subject be praise and thanksgiving, you need not look as though you were at a funeral.
Above all, let the melody of the song, be accompanied by the melody of the heart; never losing sight of the important direction of the poet, “Rehearse his praise with awe profound, Let knowledge lead the song; Nor mock him with a solemn sound, Upon a thoughtless tongue.”