Tuttle — Changes in Elevation of Land and Sea. 343 



or depressed as much as three feet from its average elevation 

 during strong gales in the above directions. 



When we pass from the daily changes to the monthly 

 averages of mean sea level, we find a remarkable correspond- 

 ence between the height of mean sea level and the movement 

 of the northwest and west winds, while the northeast and 

 easterly winds appear to have little effect on these monthly 

 averages. Probably the greater and more uniform effect of 

 the westerly winds, which has been proved by analysis for a 

 long series of years, is due to the fact that they are the prevail- 

 ing winds, last for a considerable time, have greater intensity, 

 and are effective over large areas, while the easterly winds are 

 more local, and changeable. 



The atmospheric pressure does not usually appear to have an 

 important share in determining the monthly values of mean 

 sea level, except as it influences the intensity of the prevailing 

 westerly winds. 



These monthly averages show a minimum mean sea level, 

 sharply defined in January, and a maximum mean sea level, 

 not sharply defined in August ; while the westerly winds have 

 a maximum sharply deh'ned in January, and a minimum not 

 sharply defined in August. This seasonal difference in eleva- 

 tion of mean sea level averages 0'55 ft. By means of yearly 

 averages we eliminate these seasonal fluctuations as well as 

 all astronomical effects, except that having a period of 19 

 years, which is so small in this latitude (less than 0*2 inch) 

 that it may be neglected. 



Particularly noticeable in the tidal curves is the very con- 

 siderable depression of half tide level in 1874, and the great ele- 

 vation of half tide level in 1878. When compared with the 

 curve of atmospheric pressures, it will be noticed that abnormal 

 changes of the barometric pressure occurred in those years, 

 which seem to show that they occasioned the tidal fluctua- 

 tions. 



Tidal elevations and barometric pressures at a number of 

 stations on both sides of the Atlantic show that these atmos- 

 pheric surges extended over a large area, and similarly affected 

 all tide gauges on the North Atlantic Ocean. 



In the year 1900 a marked depression in half tide level is 

 shown by the New York observations. As this same depres- 

 sion was observed at Fernandina, Fla., and other points,. we 

 may presume that it took place along the entire Atlantic Coast. 

 At New York City, it appears to have been caused by the 

 unusual strength of the N.W. and W. winds. 



In 1901 and 1902 mean sea level at New York City rose to 

 a height not before attained, which was probably due to the 

 low barometric pressure prevailing during those years. 



