May 1, 1896. 



KNOWLEDGE 



115 



as in the case of the wind waves of the sea when they 

 reach the shallowing slope of the shore. A very noticeable 

 result of this shortening of the front part of the billow is 

 that on the flood tide the interval between slack water and 

 high water is shorter than it would be if the form of wave 



were as shown in Fig. 1, and that on the ebb tide the 

 interval from high water to slack water is longer than the 

 corresponding interval on the flood tide. 



^Vhen the channel of a river narrows very rapidly the 

 front of the tide wave becomes so short and steep that a 

 hore is formed, in which, at last, we have the tide wave as 

 a visible billow. In studying the tide waves of sea and 

 ocean, and even of most rivers, we are at a disadvantage 

 from the fact that the slope of the billow is so gentle and 

 the length of the billow so great that the progress of the 

 wave crest cannot be followed by the eye, and we are apt 

 in actual observation to forget the invisible wave propaga- 

 tion while noticing the phenomena of rise and fall and of 

 currents. This difBculty, which at first besets the study 

 of the tides, is the converse of that met with in studying 

 the wind waves of the sea, in which the progress of the 

 wave crest fascinates the eye while the motions of the 

 water as the wave crest passes elude the observation. 



The River Severn af- 

 ords a fine example of the 

 bore, which is to be seen 

 also in many other rivers 

 which have a funnel- 

 shaped estuary. When 

 the channel of the river 

 is very shallow at the 

 sides, and bordered at low 

 tide by broad flats of mud 

 or sand, the phenomenon 

 is particularly fine ; the 

 wave, which moves as an 

 unbroken wall of water up 

 the deep part of the 

 channel, breaking in the 

 shallow parts, and the 

 water then rushes over the 

 flat sands in a roaring 

 surf. Our illustration 

 (Fig. 2) shows the Severn 

 Bore a few miles below 

 Gloucester, at a place 

 called Stonebench, where 

 a ledge of rock projects 



into the stream and probably causes the breaking of the 

 wave near the left bank of the river. The bore is not the 

 whole of the front slope of the tide wave, for after the bore 

 has passed the level of the water continues to rise, the cur- 

 rent flowing rapidly up stream. At a distance of about six 

 miles below (Uoucester the rise of level has been observed 

 to cease about one and a half hours after the passing of 

 the bore. At this time the current is still flowing slowly up 

 stream. In about a quarter of an hour the current turns sea- 

 ward, running slowly at first but soon gathering strength, 

 until it attains a speed seldom met with in other rivers. 



The d'/rc of the Trent is another line example of the 

 bore in a tidal river. 



Having now traced out the somewhat complicated 



connection between the rise and fall of water level and 

 the direction of current in the tide wave of rivers, we 

 are in a better position to explain the tidal currents of the 

 English Channel, and their relation to the times of high and 

 low water on the coast. 



In mid-Channel the course of events is very simUar to 

 that in a river, viz., when it is high tide there the current 

 is running its strongest up Channel ; it continues to run 

 up till half tide, then turns, and is running down Channel 

 most swiftly at low tide. Close in shore, especially in 

 bays, matters are very different. The front of the tidal 

 wave as it runs up Channel is of course curved, for the 

 sides of the Channel are shallow, and there the motion of 

 the wave is retarded. Just as in the case of ordinary 

 wind waves, the crest swings round and advances dead 

 on shore. Hence, when the tide is flowing in the Channel 

 the current 7iear the shore is running towards the shore. 

 At high tide, the crest of the wave being reached, the 

 waters close on shore can flow no further, for they are 

 stopped by the shore as by a wall ; consequently it is 

 slack water close in shore at high tide. After high tide the 

 current here turns and flows ofl' shore. At a position 

 intermediate between mid-Channel and the shore the course 

 of the currents is a compromise between these we have so 

 far described. At low water the stream is down Channel ; 

 then, as the tide rises, the current is partly down Channel 

 and partly shorewards. At half flood tide the current is 

 directly towards shore ; at high water directly up Channel, 

 then partly up Channel and partly off shore ; at half ebb 

 directly oS shore, and at low tide again directly down 



Fio. 2. — The Severn Bore from Stonebench. 



Channel : so that in the course of one complete tide the 

 current flows towards every point of the compass in turn. 



The work of the tides in the transport of sand and 

 shingle, mud and silt, does not in its details come within 

 our present scope, but a few points must be touched upon 

 which serve to illustrate the matters we have dealt with in 

 this article. 



In tidiil rivers mud and silt is deposited at each 

 portion of the tideway in succession when slack water 

 occurs there. In this respect the navigation of tidal rivers 

 presents a disadvantage as compared with rivers which 

 flow into non-tidal seas, in which the uninterrupted current 

 carries away the mud and silt. This, however, is deposited 

 off the month of the river, where the force of the current 



