210 Mr. A. Tylor on Tides and Waves. 



water). Thatis, instead of ■£% of the height of this lunar portion 

 of the tide representing the diurnal irregularity (which I con- 

 sider is the mean of the world), at Kurrachee the tides are so 

 exceptional that the diurnal irregularity amounts at low water to 

 §, and at high water to J of the total height on the average of 

 the tides for a month. This opposite action cannot be owing 

 to the position of the attracting bodies in the respective tides 

 being in the plane of the equator. 



I would observe that the deep central ocean without any 

 vertical tidal movement or tide-wave observed is certainly forty 

 times as large as the shallow coast-sea, where a rise of tide is 

 observable. The composers of figs. 1, 2, and 3, PL IV., seem 

 to regard the coast alone, which I consider the exception. 

 They do not seem to think of the tidal conditions in the great 

 mass of the ocean at all in forming their theory. 



It appears from figs. 1 and 2, PI. II., and fig. 1, PI. III., 

 from observation, that the level of the sea at high water, even 

 in the tidal estuary of the Thames, is only raised 5 feet, and in 

 that of the Clyde 1 foot, above the central ocean. 



The high-water points from Falmouth to Sheerness are nearly 

 level; they only deviate 1 foot in 500 miles from a straight line. 



The fact has not been sufficiently considered, that water in 

 open channels can be moved under certain conditions against 

 gravity, and that the great central mass of the ocean swinging 

 backwards and forwards every six hours is one of the forces that 

 can easily overcome gravity when producing a slow current. As 

 early as 1853 I gave a drawing in your Journal (p. 259) of the 

 bottom- water outside the bar of the Mississippi being raised to 

 the surface 16 feet against gravity by the current of fresh water 

 flowing outwards, partly impelled by gravity (propulsion) and 

 partly sucked or drawn by the tidal water in front of it (aspira- 

 tion) : see fig. 4, PI. III. I still believe that the tidal current 

 acts like the piston of a pump, and reduces the pressure in its 

 rear, and draws or sucks out the coast-water after it in the ebb- 

 tide, and pushes the water back again to fill up the gap when 

 its motion is reversed by the luni-solar force in the flowing tide. 

 I first observed evidence of this action on the bars of rivers, and 

 represent it in fig. 2, PI. III. As the water in the Mississippi 

 is 100 feet deep at a comparatively short distance behind the 

 bar BC, and is in motion from* top to bottom, the lower 

 water is evidently drawn over the bar and up an ascent of 84 

 feet against gravity, by the pressure of the water at B C on 

 the bar being reduced by the tide or mass of oceanic water mo- 

 ving steadily before it. Motion, of course, ensues in the direc- 



* Humphrys and Abbott record rapid motion at the bottom at Carrol- 

 ton, page 149. 



