140 POPULAR LECTURES AND ADDRESSES. 



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 instruc- 

 tion 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 noteworthy 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 

 horizontally, through a great distance. Thus the 

 primary phenomenon of the tides is after all the 

 tidal current ; and it is the tidal currents that are 

 referred to on charts where we have arrow-heads 



