National Geographic : 1899 Oct
TIDES OF CHESAPEAKE BA Y been expended in maintaining tide-measuring instruments in the North sea, along the coasts of France, and in the Mediter ranean sea. These have been connected, wherever possible, in efforts to compare the sea-level at different ports around Eu rope. France and Spain occupy favorable positions in work of this kind, since by comparatively short lines, without leaving their own territory, they may connect the mean sea-levels of the Atlantic and the inland waters east of Gibraltar. How impor tant the determination of heights is regarded abroad may be judged from the fact that up to 1895, the date of the last pub lished report of the International Geodetic Association, more than 122,000 kilometers of precise leveling had been done in continental Europe, and nearly 99,000 permanent bench-marks had been established. This work has had its greatest develop ment in Germany, Austria, and France, in the order named. The average tide for the entire bay is about one foot; possibly less. For Old Point Comfort we have two and one-half feet; for the mouth of the Potomac, one foot; for Washington, three feet; Richmond, three feet; Elk river, at the head of the bay, two feet, and Annapolis less than one foot. The wind effect, however, is sometimes more than the total tide. For example, at Baltimore the wind effect may amount to three feet, while the tide proper, uninfluenced by local disturbances, is only one third as much. This diminution in the height of the tides as we come northward from the entrance and the subsequent in crease as we continue on in the same direction is one of the peculiar features of the tidal phenomena of the bay. The small range at Annapolis is due partly to the change in width of the bay, but principally to the fact that there is an in terference at this point between the incoming and outgoing tidal waves. When the crest of the southbound movement reaches the mouth of the Severn river it meets the northbound wave from the capes, and a partial neutralization of the vertical motion of the water takes place. Another interesting point in connection with the subject is that the rate of progress of the tidal wave from the mouth of the Potomac to Washington is somewhat less than that of an ordinary steamer, so that a vessel requiring the greatest depth possible would be able to enjoy the condition of high water dur ing its entire passage up the river. The fact was first brought out by Mr C. A. Schott many years ago, when the Great Eastern, of transatlantic cable fame, availed itself of this favorable cir cumstance and came to anchor within a few miles of the Capitol.