Mr. A. Tylor on Tides and Waves. 211 



tion of least pressure ; and that happens to be, as far as the upper 

 part of the water is concerned, against gravity, and is similar to 

 what happens in a mill-race (fig. 3, PL III.}. This actiorj, 

 which takes place at the mouths of all large rivers, is a clue to 

 the tidal movements. It is true that in the Severn at Beachley 

 the high-water level of spring tides is 25 feet above the 

 Ordnance Datum, and 21 feet above the level of the central 

 ocean. But this is an exceptional case that can be explained. 

 At Beachley at high water the cross section is 400,000 sup. feet, 

 and at low water only 25,000 sup. feet (see fig. 1, PL IL). 



At Stonebench, 36 miles higher, there is only a cross section 

 of 240 feet, and a depth of 3 feet at low water. 



The cross section at Beachley is not a tenth of the section a 

 few miles lower down in the Bristol Channel. 



The exceptional height of the tide there is solely due to the 

 funnel shape of the channel, caused by the hard rocks that pre- 

 vent the tideway being excavated to the usual form. This 

 exception proves the rule. I compare the ebb-tide to the action 

 of a mill-stream, thus : — 



Fig 3, PL III., represents, from actual observation, a case 

 of water moving against gravity in an open channel, and against 

 the direction of the slope of the surface of the water. The 

 stream of water passing over the weir at B falls in a thin stream at 

 great velocity to C r Here it changes its direction and the current 

 is against the slope of the surface, viz. towards D, instead of 

 towards C y , which would be the direction of gravity. The 

 stream D E in uniform motion reduces the pressure at D and 

 draws the water from C after it — just as the great central 

 oceanic stream represented in figs. 1 and 2, PL II., and in 

 fig. 1, Plate III., draws the coast-water westwards and forms a 

 gap which is filled by the tide, the oceanic stream having reversed 

 its direction in six hours, as shown in fig. 4, PL IV., by luni- 

 solar attraction pushing on to the coast-line the flowing tide. 



My explanation of the luni-solar attraction in fig. 4, PL IV., 

 is placed adjoining the drawing. 



It will be observed I omit in the diagram (fig. 4, PL IV.) the cus- 

 tomary theoretical intumescences opposite to each other moving 

 with the moon, but through each of which every part of the earth 

 is supposed to pass daily, as in figs. 2 and 3, PL IV., and I show 

 an alternating tide in the ocean itself instead, in fig. 4. I con- 

 fess I cannot follow the supposed changes of form of the ocean- 

 surface in figs. 1, 2, and 3, PL IV., nor imagine either that such 

 movements could possibly occur, or that they would at all de- 

 scribe the tidal changes at any point of the globe as known to 

 observers. 1 remark, the writers give no dimensions of the in- 

 tumescences, or calculate the force to convert the circle a b into 



P2 



