TBANS ACTIONS OF SECTION E. 639 



constantly of smaller dimensions as it approached China, thus supplying ground 

 for the contention, which is further supported by the legends of China and 

 Japan, that the tea plant was introduced there by a Buddhist missionary from 

 India. 



6. Influence of the Straits of Dover on the Tides of the British Channel and 

 North Sea. By Sir William Thomson, F.B.S. 



The conclusions are — 



1. The rise and fall of the water surface, and the tidal streams throughout the 

 North Sea, north of the parallel of 53° (through Cromer in Norfolk and the North 

 Coast of Holland and Hanover) are not sensibly different from what they would 

 be if the passage through the Straits of Dover were stopped by a barrier. 



2. The main features of the tides (rise and fall and tidal streams) throughout 

 the British Channel, west of Beachy Head and St. Valery-en-Caux, do not differ 

 much from what they would be if the passage through the Straits were stopped 

 by a barrier between Dover and Cape Grisnez (Calais). 



3. A partial effect of the actual current through the Straits is to make the tides 

 throughout the Channel, west of a line from Hastings to the mouth of the 

 Somme, more nearly agree with what tbey would be were there a barrier along 

 this line, than what they would be if there were a barrier between Dover and Cape 

 Grisnez. 



4. The chief obviously noticeable effect of the openness of the Straits of Dover 

 on tides west of Beachy Head is that the rise and fall on the coast between 

 Christchurch and Portland is not much smaller than it is. 



5. The fact that the tidal currents to the westward commence generally an 

 hour or two before Dover high water, and to the eastward an hour or two before 

 Dover low water, instead of exactly at the times of Dover high and low water, is 

 also partially due to the openness of the Straits of Dover. 



6. The facts referred to in Nos. 4 and 5 are wholly due to two causes — (1) 

 The openness of the Straits of Dover ; (2) fluid friction (in eddies along the bottom 

 and in tide races). It is certain that (1) and it is probable that (2) is very 

 sensibly influential. Without farther investigation it would be vain to attempt 

 to estimate the proportionate contributions of the two causes to the whole effect. 



7. It is certain that were the Straits of Dover barred, and were the water 

 frictionless, there would be a nodal line (or line of no rise and fall) across the 

 Channel from somewhere near St. Alban's Head on the English coast to somewhere 

 near Cape La Hogue'or Cherbourg or Cape Barfleur on the French coast ; that 

 west of this line the time of low water, and east of this line the time of high water, 

 would be exactly the same as the time of high water at Dover ; and that through- 

 out the Channel the water would be flowing eastwards while the tide is rising at 

 Dover, and westwards while the tide is falling at Dover. 



8. Understanding from Fourier's 'Elementary Principles of Harmonic Analysis' 

 that all deviations from regular simple harmonic rise and fall of the tide within 

 twelve hours are to be represented by the superposition of simple harmonic oscil- 

 lations in six-hour period, four-hour period, three-hour period, and so on, like 

 the "overtones" which give the peculiar characters to different musical sounds of 

 the same pitch, the six-hourly oscillation which gives the double low water at Port- 

 land and the protracted duration of the high water at Havre,* is probably due to 

 the complex harmonic character of the current through the Straits of Dover — that 

 is to say, definitely, to a six-hour periodic term in the Fourier series representing the 



* From Admiralty Tide Tables for 1878, page 159:— "At Havre, on the 

 French coast, the high water remains stationary for one hour, with a rise and fall of 

 three or four inches, .... and only rises and falls thirteen inches for the space 

 of three hours ; this long period of nearly slack water is very valuable to the traffic 

 of the port, and allows from fifteen to sixteen vessels to enter or leave the docks on 

 the same tide." 



