411 



tide curve along the shore will assume a position nearly 

 parallel to the line of coast. The phenomena of the 

 motion in mid-channel will therefore be the same as that 

 in ordinary channels, and the direction of the current will 

 change only when the water is at its mean height. But 

 near the coast there will be a reflected wave from the coast, 

 and the phenomena of motion will more resemble that of 

 the tides in a short gulf; the direction of the current will 

 therefore change at high and low water. At places inter 

 mediate, intermediate phenomena will take place. Thus 

 the direction of the current will be intermediate between 

 the direction of the current at the centre of the channel 

 and that along the shore. A little consideration will show 

 that on one side, the current at any spot will, in one com 

 plete period from high to low water, go round every point 

 of the compass in the direction of the hands of a watch, 

 and on the other side in the opposite direction. The for 

 mer phenomena are found to occur on the English side of 

 the Channel, the latter on the French side. 



Such are our present theories of the tides. It may be 

 said that we have almost obtained a complete explanation 

 of river phenomena. The great difficulty is with the 

 ocean. In mathematical language we can solve the ques 

 tion of the motion of the tide in one dimension but not in 

 two. Philosophers have been of late very assiduous in 

 collecting the &quot; facts &quot; of oceanic tides, rightly considering 

 that such knowledge will enable us greatly to advance the 

 theory. Many interesting papers on this subject will be 

 found in the Philosophical Transactions, and to these we 

 must refer our readers. 



The first step was the construction of what is called 

 &quot; co-tidal lines.&quot; These are curves drawn through all the 

 places on the globe which have high water at the same 

 instant. By drawing these lines for the successive hours 

 of the day we can trace the progress of the tide wave over 

 the whole globe. A beautiful map of these will be found 

 in Johnston s Physical Atlas. It appears that the tide 

 wave travels east to west in the great South Seas, as a 



