358 PHYSICAL GEOGRAPHY, 



water at one end was four inches higher than at the other, owing to the action of the 

 wind upon it. Major Rennell states, that a piece of water, ten miles broad, and generally 

 only three feet deep, has, by a strong wind, had its waters driven to one side, and so 

 sustained as to become six feet deep, while the windward side was laid dry. In rivers 

 also, when they are swollen by the rains, the increased force of their current 

 materially affects the advance of the tide ; and when their waters are low the tidal wave 

 will proceed along their channel to a greater distance, and hold up in them during a 

 longer interval than under opposite circumstances. The astronomer Flamstead, who had 

 frequent occasion to go to Greenwich by water, turned his attention to the subject of the 

 tide in the Thames ; and he remarks : " When, by reason of great droughts in summer, or 

 extreme frosts in winter, the springs are low, and the fresh waters less than usual, the 

 tides may hold up longer than the times noted in the table ; also when strong north- 

 westerly or northerly winds blow, which bring in an extraordinary flood from the northern 

 seas, and keep it up longer than other times ; so, on the contrary, when the winds blow 

 hard on the opposite points of the- compass, or when we have much rain and great freshes, 

 the tides hold not out so long as the times shown in the table, the freshes overpowering 

 and checking them sooner ; yet have I never found that the difference between the calcu- 

 lated and observed high waters have much exceeded half an hour most commonly they 

 are scarce half so much." There are some singular circumstances connected with the 

 tides of rivers which have thus been noticed by a recent writer: "They are not of 

 equal duration, as is the case in most parts of the sea ; but the ebb tides frequently last 

 twice as long as the flowing tides. At Rotterdam the tide flows for about four hours and 

 five minutes, but the ebb lasts seven hours and fifty-five minutes. The Meerwede at 

 Dortrecht flows against the current of the river for three hours and fifty-one minutes, 

 and with it eight hours and nine minutes. This difference is easily explained when the 

 force of the river current is taken into account. The same circumstance explains the 

 difference in the velocity of the ebbing and flowing tide. Between the North Sea and 

 Hamburgh the flowing tide takes five minutes to run up a mile, but the ebb tide performs 

 the same distance in less than four minutes.*. But it is difficult to explain the well- 

 established fact that the tides advance much farther into a river than might be expected. 

 When the tide at the mouth of a river rises four feet, we might suppose that it would 

 advance only to such a point in the river where the surface is four feet above the sea ; but 

 it has been ascertained that it advances farther. It seems that the volume of water which 

 is carried up by the tide is pushed onwards by the mass behind it, and carried to a 

 greater distance than the inclination of the river bed would seem to allow. It has also 

 been observed, that during the flowing of the tide the surface of the water in the river 

 presents a somewhat convex form, the water along the banks being a little lower than in 

 the middle of the river, and that during the ebb the contrary takes place. The flowing 

 tide raises the water from below, and thus sooner affects the main body of the river, 

 where it has more room to operate than the water near the margin. In accordance with 

 this explanation, it is observed that the flowing tide is perceptible in the middle, while it 

 is still ebbing along the banks, and that vessels which are at anchor near the banks are 

 turned round before the water on the surface of the river near the banks begins to flow 

 upward." 



The change produced in the aspect of rivers by the advance of the tide is of the most 

 striking description, and confers important advantages upon the towns seated along their 

 banks, rendering them essentially maritime, though at a considerable distance from the 

 sea. The Avon at Bristol supplies a remarkable example of the alteration, and of the 

 commercial benefits resulting from it. Its natural character at St. Vincent's rocks is that 

 of a shallow brawling stream, scarcely navigable by the smallest craft ; but upon the flow 



