THE TIDES 



[EVENING LECTURE TO THE BRITISH ASSOCIA- 

 TION AT THE SOUTHAMPTON MEETING, 

 FRIDAY, AUGUST 25, 1882] 



THE subject on which I have to speak this evening is 

 the tides, and at the outset I feel in a curiously diffi- 

 cult position. If I were asked to tell what I mean by 

 the Tides I should feel it exceedingly difficult to answer 

 the question. The tides have something to do with mo- 

 tion of the sea. Rise and fall of the sea is sometimes 

 called a tide ; but I see, in the Admiralty Chart of the Firth 

 of Clyde, the whole space between Ailsa Craig and the 

 Ayrshire coast marked " very little tide here." Now, we find 

 there a good ten feet rise and fall, and yet we are authori- 

 tatively told there is very little tide. The truth is, the word 

 " tide " as used by sailors at sea means horizontal motion of 

 the water; but when used by landsmen or sailors in port, 

 it means vertical motion of the water. I hope my friend 

 Sir Frederick Evans will allow me to say that we must 

 take the designation in the chart, to which I have referred, 

 as limited to the instruction of sailors navigating that part 

 of the sea, and to say that there is a very considerable 

 landsman's tide there a rise and fall of the surface of the 

 water relatively to the land though there is exceedingly 

 little current. 



One of the most interesting points of tidal theory is the 

 determination of the currents by which the rise and fall is 

 produced, and so far the sailor's idea of what is most note- 

 worthy as to tidal motion is correct: because before there 

 can be a rise and fall of the water anywhere it must come 

 from some other place, and the water cannot pass from 

 place to place without moving horizontally, or nearly hori- 



287 



