a channel separating the Orkney Islands from the coast of Caithness, and joining the North Sea and the Atlantic. It is about 17 miles long from E. to W., and 6 or 8 broad. At the eastern extremity, 4 miles N.E. of Duncansby Head, are the Pentland Skerries, two islands, the largest of which has a lighthouse, with two fixed lights. The depth of the strait varies from 10 to 40 fathoms; and the current runs strongly to the E., with a velocity from 4 to 9 miles an hour.