p is a perfect mute, having no sound.
b an imperfect mute, having proper sound, but limited.
m a perfect consonant, having sound, and continued.
f an imperfect consonant, having no sound, but audible.
In the third position we have th as heard in the words then and thin, formed by placing the tip of the tongue between the teeth, and pressing it against the upper teeth. The only difference between these articulations is, that like f and v, the one is formed by the breath only, and the other by the breath and voice together[5].
Here also may be distinguished the perfect and the imperfect consonant; for the th in thin has no sound, but is audible, whereas the th in this, there, has a sound, and that continued[6].
In the fourth position there are several consonants formed.
1st, If the breath be stopped, by applying the fore part of the tongue forcibly to that part of the palate, which is contiguous to the fore teeth, we produce the perfect mute t, having neither aspiration nor guttural sound. By accompanying this operation of the tongue and palate with the guttural sound, we shall pronounce the letter d, which, like b, of the first position, may be considered as a mute, though not perfect. For in pronouncing ed, the tongue at first gently touches the gum, and is gradually pressed closer, till the sound is obstructed; whereas in pronouncing et, the tongue is at once pressed so close, that the sound is instantly intercepted.
2dly, If the tip of the tongue be turned up towards the upper gum, so as not to touch it, and thus the breath be cut by the sharp point of the tongue passing through the narrow chink left between that and the gum, we pronounce the sibilating sound of s. If we accompany this operation with a guttural sound, as in b, v, and th in then, we shall pronounce the letter z; the same difference subsisting between s and z as between f and v, p and b, tħ and th.
3dly, If we make the tip of the tongue vibrate rapidly between the upper and lower jaw, so as not to touch the latter, and the former but gently, we shall pronounce the letter r. The more closely and forcibly the tongue vibrates against the upper jaw, the stronger will the sound be rendered. It is formed about the same distance from the teeth as the letter d, or rather somewhat behind it.