144 POPULAR LECTURES AND ADDRESSES. 



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 attrac- 

 tions 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 



FIG. 1 8. Wave forms. 



English Channel, and the long Atlantic wind 

 waves, and the great swell of the ocean from one 

 hemisphere to the other and back again (under 

 the name which I find in the harmonic reduction 

 of tidal observations), proved to take place once a 

 year, and which I can only explain as the result 

 of the sun's heat. 



But while the action of the sun's heat by means 



