Bos`phorus (Ox-ford), a channel 17 m. long and from 3 to ½ m. broad, and about 30 fathoms deep, strongly defended by forts, extending from the Sea of Marmora to the Black Sea; subject to Turkey. It derives its name from the channel which, according to the Greek myth, Zeus, in the form of an ox, crossed into Europe with Europa on his back.