A history of the English Language

178.
Weakening of Unaccented Vowels.
A little observation and reflection shows us that in unaccented syllables, too, the spelling 
does not accurately represent the pronunciation today. This is because in all periods of 
the language the vowels of unstressed syllables have had a tendency to weaken and then 
often to disappear. This is true of all parts of a word. For example, we do not distinguish 
in ordinary or rapid speech between the vowels at the beginning of 
ago, upon, opinion
. 
The sound in all three words is [
ə
]; in other cases it is commonly [
ə
], or [I]. Consider the 
unstressed middle or final syllable in the words 
introduce, elegant
[
ə
, I], 
drama, color, 
kingdom, breakfast
(br
ε
kf
ə
st 
or
br
ε
kf’st), 
Monday
[i]. The weakening is especially 
noticeable in words from French where an accented vowel came to be unaccented in 
English (cf. French 
mouton, raisin, bonté
with English 
mutton, raisin, bounty
). One must 
not be misled by the spelling. The original spelling was generally retained and in recent 
times has occasionally influenced the pronunciation so that the quality of the vowel has 
been restored to something like its earlier character. 
Window
now has a fairly well-
defined diphthong in the final syllable [oU] or [o
u
], but the weakened vowel is evident in 
the dialectal pronunciation 
winder
. Misguided purists often try to pronounce the final 
syllable of 
Monday,
with the full quality of the diphthong in 
day
. But even when the 
vowel has been restored in standard speech the weakened form is generally apparent in 
informal speech and in the dialects.