13.  The vowel of foot (see 10) became “open” in quality and shorter in quantity, i.e., it fell together with the old short u-vowel of words like full, wolf, wool.  This change has taken place in a number of words with an originally long u (Chaucerian long close o), such as forsook, hook, book, look, rook, shook, all of which formerly had the vowel of boot.  The older vowel, however, is still preserved in most words of this class, such as fool, moon, spool, stoop.  It is highly significant of the nature of the slow spread of a “phonetic law” that there is local vacillation at present in several words.  One hears roof, soot, and hoop, for instance, both with the “long” vowel of boot and the “short” of foot.  It is impossible now, in other words, to state in a definitive manner what is the “phonetic law” that regulated the change of the older foot (rhyming with boot) to the present foot.  We know that there is a strong drift towards the short, open vowel of foot, but whether or not all the old “long oo” words will eventually be affected we cannot presume to say.  If they all, or practically all, are taken by the drift, phonetic law 13 will be as “regular,” as sweeping, as most of the twelve that have preceded it.  If not, it may eventually be possible, if past experience is a safe guide, to show that the modified words form a natural phonetic group, that is, that the “law” will have operated under certain definable limiting conditions, e.g., that all words ending in a voiceless consonant (such as p, t, k, f) were affected (e.g., hoof, foot, look, roof), but that all words ending in the oo-vowel or in a voiced consonant remained unaffected (e.g., do, food, move, fool).  Whatever the upshot, we may be reasonably certain that when the “phonetic law” has run its course, the distribution of “long” and “short” vowels in the old oo-words will not seem quite as erratic as at the present transitional moment.[154] We learn, incidentally, the fundamental fact that phonetic laws do not work with spontaneous automatism, that they are simply a formula for a consummated drift that sets in at a psychologically exposed point and gradually worms its way through a gamut of phonetically analogous forms.

[Footnote 154:  It is possible that other than purely phonetic factors are also at work in the history of these vowels.]

It will be instructive to set down a table of form sequences, a kind of gross history of the words foot, feet, mouse, mice for the last 1500 years:[155]

[Footnote 155:  The orthography is roughly phonetic.  Pronounce all accented vowels long except where otherwise indicated, unaccented vowels short; give continental values to vowels, not present English ones.]