32 



PROCEEDINGS OF THE AMERICAN ACADEMY 



is the result of about £00 nriporvfrT'OTiF! of h?<rh wjitpt nnd a« many of 

 low water. The only causes which have bee:> . : _ . ed to affect the 

 mean level are a small astronomical term depending upon the sun's 

 declination, the variation of atmospheric pressure, and the winds. The 

 effect of the astronomical term is equal to a cos 2 /, in which a is a func- 

 tion of the latitude of the port, and I is the sun's longitude. The effect 

 of the variation of atmospheric pressure is equal to 13 m .568 (P — p), 

 in which p is the observed barometric pressure of the atmosphere in 

 meters, and P the annual mean pressure. The coefficient 13 m .568 is 

 assumed upon the hypothesis that the variation of the mean level of 

 the ocean at any place is to the variation of the barometric column as 

 the specific gravity of mercury is to that of water. M. Dausy ob- 

 tained 15 m .5 for this coefficient from 150 observations of the tides and 

 of atmospheric pressure, made at Lorient, but Mr. Lubbock from ob- 

 servations at Liverpool obtained only 11.1 for that port. The theoreti- 

 cal value is perhaps more nearly correct than either of the other two. 

 In the second numerical column of the following table is given the 

 atmospheric pressure at Paris in millimetres for each month of the 

 year, taken from Kaemtz's Meteorology, which is assumed to vary but 

 little from the pressure at Brest, so that it may be used in computing 

 the effect of the variation of pressure upon the mean level of the sea 

 at that port. The following variations of mean level corrected for the 

 astronomical term and variation of atmospheric pressure must be the 

 effect of the winds, and other causes. The value of a in the astronomi- 

 cal term for the latitude of Brest is — .007. The corrections and the 

 corrected monthly mean heigths of the ocean are also given in the fol- 

 lowing table. 



