But every other part of the county -- at least every part conveniently reached from the railroads -- is also highly esteemed for select residence purposes; and, indeed, Westchester County throughout its extent is peculiarly a residential county. Spuyten Duyvil Creek and the Harlem River, which separate Manhattan Island from the mainland and form a portion of the southern boundary of the old County of Westchester, are in reality only an arm of the sea: and though to the superficial observer they may appear to constitute one of the mouths of the Hudson, they have no such function, and, indeed, receive none of its flow. The two are strictly to be considered not as a river, but as a strait, connecting the tide waters of the East River and Sound with those of the North River. Their length is about eight miles. The Harlem River at its eastern extremity is divided by Randall's Island into two channels -- the southern and principal one communicating with Hellgate, and the northern one (unnavigable), called the Bronx Kills, passing between the island and the Westchester shore into Long Island Sound. The Harlem and Spuyten Duyvil waterway presents the remarkable phenomenon of double tides, which vary decidedly iu height, time of occurrence, duration of rise and fall, and swiftness of flow. ''The

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tides in the Harlem River," says General John Newton, in a report to the War Department, " are chiefly due to the propagated Hellgate wave, while the latter is the result of the contact of the Sound and Sandy Hook tides. The tides in the Hudson River and Spuyten Duyvil are produced by the propagation of the sea. tide through the Upper and Lower bays." The mean rise of the tide in the Harlem is from Ave and one-half to six feet; in the Spuyten Duyvil Creek it is three and eight-tenths feet.