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"The Hebrews divided the night into three watches, the Greeks usually into four (sometimes five), the Romans (followed by the Jews in New Testament times) into four." [OED]The meaning "small timepiece" is from 1588, developing from that of "a clock to wake up sleepers" (1440). Watchmaker is recorded from 1630; watchtower is attested from 1544.
the periods into which the time between sunset and sunrise was divided. They are so called because watchmen relieved each other at each of these periods. There are frequent references in Scripture to the duties of watchmen who were appointed to give notice of the approach of an enemy (2 Sam. 18:24-27; 2 Kings 9:17-20; Isa. 21:5-9). They were sometimes placed for this purpose on watch-towers (2 Kings 17:9; 18:8). Ministers or teachers are also spoken of under this title (Jer. 6:17; Ezek. 33:2-9; Heb. 13:17). The watches of the night were originally three in number, (1) "the beginning of the watches" (Lam. 2:19); (2) "the middle watch" (Judg. 7:19); and (3) "the morning watch" (Ex. 14:24; 1 Sam. 11:11), which extended from two o'clock to sunrise. But in the New Testament we read of four watches, a division probably introduced by the Romans (Matt. 14:25; Mark 6:48; Luke 12:38). (See DAY.)