392 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. 



