THE TIDES 



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usually caused by earthquakes. But we are apt to call any 

 not very short-time rise and fall of the water a tide, as 

 when standing on the coast of a slanting shore where there 

 are long ocean waves, we see the gradual sinkings and ris- 

 ings produced by them, and say that it is a wave we see, 

 not a tide, till one comes which is exceptionally slow, and 

 then we say " that is liker a tide than a wave." The fact is, 

 there is something perfectly continuous in the species of 



FIG. 122 Wave forms 



motion called wave, from the smallest ripple in a musical 

 glass, whose period may be a thousandth of a second to a 

 "lop of water" in the Solent, whose period is one or two 

 seconds, and thence on to the great ocean wave with a 

 period of from fifteen to twenty seconds, where end the 

 phenomenon which we commonly call waves (Fie. 122), and 

 not tides. But any rise and fall which is manifestly of 

 longer period, or slower in its rise from lowest to highest, 

 than a wind wave, we are apt to call a tide; and some 

 of the phenomena that are analysed for, and worked out 

 in this very tidal analysis that I am going to explain, 

 are in point of fact more properly wind waves than true 

 tides. 



Leaving these complicated questions, however, I will make 

 a short cut, and assuming the cause without proving it, 

 define the thing by the cause. I shall therefore define tides 

 thus: Tides are motions of water on the earth, due to the 

 attractions of the sun and of the moon. I cannot say tides 

 are motions due to the actions of the sun and of the moon ; 

 for so I would include, under the designation of tide, every 

 ripple that stirs a puddle or a millpond, and waves in the 

 Solent or in the English Channel, and the long Atlantic 



HC WL. XXX J 



