Samuel Johnson - A Grammar of the English Tongue стр 5.

S, single at the end of words, has a grosser sound, like that of z, as trees, eyes, except this, thus, us, rebus, surplus.

It sounds like z before ion, if a vowel goes before it, as intrusion; and like s, if it follows a consonant, as conversion.

It sounds like z before e mute, as refuse, and before y final, as rosy; and in those words, bosom, desire, wisdom, prison, prisoner, present, present, damsel, casement.

It is the peculiar quality of s, that it may be sounded before all consonants, except x and z, in which s is comprised, x being only ks, and z a hard or gross s. This s is therefore termed by grammarians suæ potestatis litera; the reason of which the learned Dr. Clarke erroneously supposed to be, that in some words it might be doubled at pleasure. Thus we find in several languages.

Σβεννυμι, scatter, sdegno, sdrucciolo, sfavellare, σφιγξ, sgombrare, sgranare, shake, slumber, smell, snipe, space, splendour, spring, squeeze, shrew, step, strength, stramen, stripe, sventura, swell.

S is mute in isle, island, demesne, viscount.

T has its customary sound; as take, temptation.

Ti before a vowel has the sound of si as salvation, except an s goes before, as question; excepting likewise derivatives from words ending in ty, as mighty, mightier.

Th has two sounds; the one soft, as thus, whether; the other hard, as thing, think. The sound is soft in these words, then, thence, and there, with their derivatives and compounds, and in that, these, thou, thee, thy, thine, their, they, this, those, them, though, thus; and in all words between two vowels, as, father, whether; and between r and a vowel, as burthen.

In other words it is hard, as thick, thunder, faith, faithful. Where it is softened at the end of a word, an e silent must be added, as breath, breathe; cloth, clothe.

V has a sound of near affinity to that of f, as vain, vanity.

From f in the Islandick alphabet, v is only distinguished by a diacritical point.

Of w, which in diphthongs is often an undoubted vowel, some grammarians have doubted whether it ever be a consonant; and not rather as it is called a double u, or ou, as water may be resolved into ouater; but letters of the same sound are always reckoned consonants in other alphabets: and it may be observed, that w follows a vowel without any hiatus or difficulty of utterance, as frosty winter.

Wh has a sound accounted peculiar to the English, which the Saxons better expressed by hw, as, what, whence, whiting; in whore only, and sometimes in wholesome, wh is sounded like a simple h.

X begins no English word: it has the sound of ks, as axle, extraneous.

Y, when it follows a consonant, is a vowel; when it precedes either a vowel or a diphthong, is a consonant, as ye, young. It is thought by some to be in all cases a vowel. But it may be observed of y as of w, that it follows a vowel without any hiatus, as rosy youth.

The chief argument by which w and y appear to be always vowels is, that the sounds which they are supposed to have as consonants, cannot be uttered after a vowel, like that of all other consonants; thus we say tu, ut; do, odd; but in wed, dew; the two sounds of w have no resemblance to each other.

Z begins no word originally English; it has the sound, as its name izzard or s hard expresses, of an s uttered with a closer compression of the palate by the tongue, as freeze, froze.

In orthography I have supposed orthoepy, or just utterance of words, to be included; orthography being only the art of expressing certain sounds by proper characters. I have therefore observed in what words any of the letters are mute.

Most of the writers of English grammar have given long tables of words pronounced otherwise than they are written, and seem not sufficiently to have considered, that of English, as of all living tongues, there is a double pronunciation, one cursory and colloquial, the other regular and solemn. The cursory pronunciation is always vague and uncertain, being made different in different mouths by negligence, unskilfulness, or affectation. The solemn pronunciation, though by no means immutable and permanent, is yet always less remote from the orthography, and less liable to capricious innovation. They have however generally formed their tables according to the cursory speech of those with whom they happened to converse; and concluding that the whole nation combines to vitiate language in one manner, have often established the jargon of the lowest of the people as the model of speech.