ISO POPULAR LECTURES AND ADDRESSES. 



to attraction, and how much to heating effect 

 directly or indirectly, whether on water, or on air, 

 or on water as affected by air. As to the lunar 

 tides we are quite sure of them ; they are gravita- 

 tional, and nothing but gravitational ; but I hope 

 to speak later of the supposed relation of the moon 

 to the weather, and the relation that has to the 

 tides. 



I have defined the tides as motions of water on 

 the earth due to the attractions of the sun and of 

 the moon. How are we to find out whether an 

 observed motion of the water is a tide or is not a 

 tide as thus defined ? Only by the combination of 

 theory and observation : if these afford sufficient 

 reason for believing that the motion is due to 

 attraction of the sun or of the moon, or of both, 

 then we must call it a tide. 



It is curious to look back on the knowledge of 

 the tides possessed in ancient times, and to find as 

 early as two hundred years before the Christian era 

 a very clear account given of the tides at Cadiz. 

 But the Romans generally, knowing only the 

 Mediterranean, had not much clear knowledge of 



