SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 455 



axis of the hurricaue had passed St. Thomas. la Professor Dove's 

 paper, the statement is made that the hurricaue be;?an at o^ o'clock 

 and ceased at llj, which statement, however, is supposed to have 

 been made only by estimate; at all events, experience in this matter 

 seems to correspond pretty well with what is obtaiued by computation. 

 If, in conclusion, we seek the velocity with which the outer portion of 

 the mass rotated, we find, accordiog to formula (1), bj' therein making 

 w = JS, that the required velocity was 84.5 feet = 27. •! meters per sec- 

 ond, or 12.G miles per hour. If this velocity is compared with the pro- 

 gressive velocity of the hurricane, which we have found to be about 6 

 miles per hour, it is seen that the rotating mass advanced with a rapid- 

 ity which was about half as great as its exterior velocity of rotation. 

 This circumstance points to another which we may observe when a cur- 

 rent of water runs, for instance, through the arch of a bridge into some 

 still water below the bridge. In such a case, I have had occasion to 

 observe that, between the current and the still water, eddies are formed, 

 which move forward, as if rolled by the passing current of water toward 

 the still water, and I have found that the advancing velocity of the 

 mass was pretty nearly half as great as its outer velocity of rotation, 

 which latter was about equal to the velocity of the passing current of 

 water. 



I have, unfortunately, not been able to study the movement of 

 rotating masses of water more thoroughly; but since it seems that the 

 laws governing their movement harmonize with those which govern the 

 movement of atmospheric whirlwinds, I think that a careful examina- 

 tion of rotating masses of water would, in all probability, throw much 

 light upon the movements of hurricanes. 



Let us now consider a fluid, water for instance, which is considered as 

 rotating uniformly around a vertical axis; but let us disregard the rota- 

 tion, just as we usually disregard the rotation of the earth with which 

 we move. There will then be an equilibrium at each of the level 

 surfaces of the fluid, and each particle that is upon such a level surface 

 will, when no other forces disturb the equilibrium, be at rest, independ- 

 ently of the rotation. If we now imagine ourselves to be sailing in a 

 boat upon the free surface of the water, or upon any level parallel 

 thereto, for instance, upon the level which is designated in .Fig. 1 by the 

 letters a «i bi /?, while the boat is supposed to rotate with the velocity of 

 the water, it is clear that, since the level surfaces are. normal to the 

 resultant of the acting forces, we may sail over the level surfiice in ques- 

 tion just as easily as we can sail upon a horizontal surface of still water, 

 which is acted upon only by the force of gravity. 



If this is the case, and if we next consider a body of water which, 

 being contained in a reservoir, has a horizontal surface, upon which ouly 

 the force of gravity is uniform, and imagine a new mass of water to be 

 let into the reservoir at a certain point, the previous equilibrium is 

 destroyed by the entering water, and does not return until the water 



