The reason of it is manifest from the effect which the Article has in
these phrases: it means a small or great number collectively taken, and
therefore gives the idea of a Whole, that is, of Unity. Thus likewise
_a hundred_, _a thousand_, is one whole number, an aggregate of many
collectively taken; and therefore still retains the Article _a_, tho’
joined as an Adjective to a plural Substantive: as, _a hundred years_;[3]
“For harbour at _a thousand doors_ they knock’d;
Not one of all _the thousand_, but was lock’d.”