306 PROCEEDINGS OF THE AMERICAN ACADEMY 



confirmed by all observations made in points open to the ocean. In 

 contracted estuaries and along the shore the currents follow the shore 

 line. In some points of a complicated and large bay a change in the 

 reverse direction is sometimes observed, as these points are reached 

 by the currents after many reflections from the shore. 



4. The relation of the rise of the tide to the velocity of the flow 

 solely depends upon local circumstances. If the shore extends perpen- 

 dicularly to the direction of the flow at its greatest rate, then high water 

 occurs soon after the time of maximum of velocity. In bays stretching 

 considerably inland, when the velocity is small and the rise depends 

 on the mass of water remaining in, the time of high water occurs con- 

 siderably later than the time of greatest velocity of the current from 

 the ocean. This delay becomes an essential element in the theory of 

 tides, for it determines the time and height of high water, and upon it 

 depends the retard of the spring and neap tides after the days of 

 syzygy and quadrature. 



5. The first part of the establishment is drawn from the theory ; the 

 second, with the magnitude of the greatest velocity of flow, determines 

 the influence of local circumstances. 



6. The absence of full uniformity in the mean level immediately 

 proves that the height of the lunisolar tides is not equal to the alge- 

 braical addition of the lunar and solar tides (one of the evident infer- 

 ences of theory of tidal motion). In fact, Mr. Airy deduced from the 

 observations " that the mean level is higher in the large tides than in 



the small ones." (" Tides and Waves," p. 374 " The mean 



level at Sheerness is higher in spring tide than in the neap tide 

 by seven inches nearly." .... And I inferred from this that the 

 lunisolar tide is greater than the addition of solar and lunar tides at 

 Sheerness by about fourteen inches.) This inequality might be con- 

 siderable ; but the various resistances to tidal motion on the coast re- 

 duce the large tides far more than the small tides. Proceeding from 

 the observations made in Ireland, Mr. Airy alluded to the difference 

 of the mean height of the sea round the island. The definition of the 

 normal level on the coast is immediately deduced from my explanation 

 of the phenomena of tides. In some points of a complicated large 

 bay, the level of low water at spring tide may be higher than the ocean 

 level (in case of a constant movement of the waters) ; but the level 

 in the bay during a quiescent state of waters (as observed at low water) 

 will never fall lower than the ocean level. The small motion of the 



