114 



KNOWLEDGE 



[May 1, 1896. 



cross section in order to maintain the hillock upon tlie 

 surface. In the deep waters of the open ocean the heif,'ht 

 of the tide billow is very small — say, from one to three feet. 

 As the wave approaches the shallowing slope of the shore 

 the front of tho billow is retarded wliile the hinder parts 

 press on, and the height of the wave is thereby increased. 

 The height of tho wave is also increased when it roaches a 

 narrowing channel in which the billow is laterally com- 

 pressed. Thus the highest tides arc not in the open ocean 

 where the tides are generated, but in the distant channels, 

 bays, and inlets, where the tide wave penetrates after the 

 moon has left it to run its course. Once every twelve 

 hours, as we have seen, the moon raises a billow and 

 drives or drags it forward as ;i pulse of water in the 

 southern oceans, much as the heart, in its rhythmic beat, 

 drives the blood-pulse into the arteries. r>ut, whereas the 

 blood-pulse runs the whole length of the artery, and is lost 

 in the capillaries before the next stroke of the heart 

 sends out a second pulse, the tide wave, on the other 

 hand, has not reached the most distant shores when 

 the next succeeding pulse of water is set going by the moon. 

 The tide wave of which the crest may round the Cape of 

 Good Hope at noon, travels up the west coast of Africa 

 and reaches the Azores about midnight. By this time it 

 is followed by a second pulse, which is now rounding the 

 Cape. It is four o'clock in the morning before the 

 crest of the first wave has reached the entrance of the 

 English Channel, where the shallower water makes it 

 travel slower. It takes another six hours to run the length 

 of the Channel, reaching the Straits of Dover at 10 a.m., 

 where the height of the wave is considerable, owing, 

 partly, to the narrowing of the Channel. Thus, at 

 Dover the water rises twenty-one feet, and this occurs 

 at Dover about six hours after the wave crest passed the 

 Land's End. Now, the next incoming tide billow on 

 its way from the Southern Ocean has not yet got as far as 

 the Azores, and the water at the entrance to the Channel 

 is at a low level — this being, in fact, the position of the 

 trough between two succeeding tide billows. Therefore, 

 when we have the water heaped up (and pent up, too, 

 for the Straits are narrow) near Dover, the water being 

 low at the other end of the Channel, the action of the 

 earth's gravity sets a long wave travelling back from 

 Dover towards the Land's End ; and this is the ebb tide 

 in the English Channel. The time which a free long 

 wave takes to travel the length of the Channel being 

 about six hours, the return wave will at 4 p.ji. have 

 emptied the Channel of nearly as much water as was sent 

 in by the tidal billow. 



This is just twelve hours since the last tidal billow 

 arrived at the mouth of the Channel, and at this moment 

 the next billow comes running in from the South Atlantic, 

 and " the tide " again runs up Channel. Thus the natural 

 period for the passage of a free long wave up and down 

 the Channel coincides with the interval between two tide 

 pulses. Were it otherwise — if, for instance, the tide were 

 ebbing from Dover, and at the same time the crest of the 

 succeeding pulse were, say, at Portland — the rise and fall 

 of water in the Channel would be much less than is 

 actually the case. As it is, what happens in the Channel 

 can be well illustrated by sending a wave along a trough 

 by tipping up one end, and keeping the wave going back- 

 wards and forwards by again gently tipping the same end of 

 the trough just as the wave returns to where it started from. 

 It will be noticed then that there is a considerable rise 

 and fall of water at each end of the trou,i;h, with little 

 horizontal motion there ; at the middle point there is 

 hardly any rise and fall, but a considerable horizontal 

 swing of the water. The same thing happens in the 



Channel. The rise at Dover is twenty-one feet ; at Portland 

 the rise and fall is scarcely perceptible, but here are the 

 strongest tidal currents, forming the well-known Portland 

 Race, which rushes (eastward on the Hood and westward 

 on the ebb) past the rocky promontory, leaping and 

 foaming over tho "Ledge" in a wild turmoil of w/iter, 

 dreaded by the mariner, but fascinating to the watcher of 

 waves — when he is on ti mi firma. We shall have more to 

 say about the Portland llacc in our next article, when we 

 are dealing with waves in running water. 



We traced the tidal billow on its rapid course up the 

 South Atlantic as far as the south-western coasts of the 

 Lritish Isles. Here the wa\o divides, and while one part 

 runs up the English Channel as we have described, 

 and is nearly, though not quite, stopped at the Straits 

 of Dover, the other half takes a western course, swings 

 round the North of Scotland, and travels comparatively 

 slowly— say forty miles an hour — along our eastern coasts 

 in a southerly direction. It is this part of the original 

 tidal billow which furnishes the greater portion of the 

 wave which once in every twelve hours enters the tideway 

 of the Thames. In this comparatively short tideway of, 

 say, seventy miles, there is never more than one wave 

 crest at a time, the free long wave running up to 

 Teddington Lock and back again to the entrance of the 

 tideway before the next tidal billow reaches the Nore. 

 In a long tideway, such as that of the Amazon, the case 

 is different, for there are at any moment a number of wave 

 crests travelling up the channel. They enter the mouth 

 of the tideway at intervals of twelve hours, and follow one 

 another up the river as a train of "solitary" waves; but 

 between each pair of crests is, not a fiat surface, but a 

 trough, which is the negative, or inverted, crest of the nega- 

 tive, or ebb, tide wave. Where the billows or hillocks are, 

 the current runs up stream ; where the hollows are, the 

 water runs down stream. 



Neglecting, at first, the effects of the shallowing of the 

 river channel and of friction, we may represent by Fig. 2 

 the wave of flood tide followed by the wave of ebb tide, 

 but necessarily with an exaggerated height ; the arrows 

 show the direction of the flow of water during flood and 

 ebb tide. Let us deal with the tide in the Thames, and 

 suppose an observer to place himself at London Bridge. 

 When the point A is opposite to him it is slack water ; 

 when A has passed the current begins to flow up stream — 

 slowly at first, and afterwards more quickly, the level of 

 the water on the banks rising the whole time. AVhen 15, 

 the crest of the wave, is opposite, it is high water at London 

 Bridge, and the up-stream current is at its strongest. When 

 high water has passed the current continues to jloir up stream, 

 but at diminishing speed, the level of the u'alcr fnllimj all tlie 

 while, until when the point C is opposite the post of obser- 

 vation slack water is reached. The current then turns, 

 and flows down stream, slowly at first but gaining speed, 

 the level of the water continuing to sink until the point D 

 is opposite, when it is low water and the stream of ebbing 

 tide is running its fastest. From that point the level of 

 water begins to rise while the current continues to run 

 down stream. When the point E is opposite the post of 

 observation it is again slack water and the tide current is 

 on the turn; the point E corresponds with point A, and 

 the cycle of phenomena which we have described begins 

 again. 



The effect produced by the shallowing and narrowing of 

 the river as we proceed up stream, and by friction with 

 the banks and bottom of the channel, is to retard the 

 progress of the front of the wave as it travels up stream 

 while the hinder part of the billow presses on, so that the 

 front of the billow becomes steeper than the back, much 



