OF ARTS AND SCIENCES : OCTOBER 10, 1865. 35 



inclusive, and those tending to depress it, for the other six months, 

 from September to February inclusive." By referring to the preced- 

 ing curve representing the variation of mean level at Key West, it is 

 seen that these winds cannot cause the corrected variation of mean 

 level, since the argument of variation of the former does not at all cor- 

 respond with that of the latter, and hence there must be still some 

 other cause affecting the mean level of the sea at Key West also. 



From what has been stated, it is evident that the effect of the winds 

 at Brest must be to decrease the amount of variation, and to cause the 

 maximum and minimum to happen a little later in the season of the 

 year. On the other hand, their effect at Key West must increase the 

 variation and cause the maximum and minimum to happen earlier in the 

 season of the year. If, therefore, the variations of mean level at Brest 

 and Key West were corrected for the effect of the winds, the argu- 

 ments of the variations at both ports would probably be the same, 

 raakinff the maxima and minima of the variations about October and 

 April. This indicates that the cause or causes of the variation of mean 

 level which we have yet to seek are not local, but more general, affect- 

 ing both ports simultaneously. There have been no researches to show 

 that the argument of the corrected variation of mean level would be 

 about the same at other ports of the Atlantic ocean also. 



There is still another cause affecting the mean level of the ocean at 

 different seasons, which is much more effective than either of those 

 which have been stated, and which, I think, satisfactorily accounts for 

 the remaining and greater part of the observed variation, which has 

 not been explained. This is a tangential force arising from the mo- 

 tions of the ocean combined with the motion of the earth's rotation. It 

 was first brought out in its most general form in my paper on the 

 "Motions of Fluids and Solids relative to the Earth's Surface," pub- 

 lished in the Mathematical Monthly, and also in an abridged form in 

 Silliman's Journal (Second Series, Vol. XXXI.), and expressed in the 

 following language : In whatever direction a body moves on the surface of 

 the earth, there is a force arising from the earth's rotation ivhich deflects 

 it to the right in the northern hemisphere, but to the left in the southern 

 hemisphere. From this force thei'e must arise a change in the level of 

 the sea wherever its waters have a motion of any kind, and as these 

 motions depend, for the most part, upon the difference of temperature 

 of the ocean between the equator and the poles, and consequently upon 

 the change of seasons, there must be a change in the mean level of the 



