THE TIDES. 149 



little rise and fa!!, as the water is kept heaped up 

 and does not sink by anything like its usual amount 

 from the extra high level that it has at high water. 

 But I fear I have got into questions which are 

 leading me away from my subject, and as I 

 cannot get through them I must just turn back. 



Now think of the definition which I gave of the 

 "tides," and think of the sun alone. The action of 

 the sun cannot be defined as the cause of the solar 

 tides. Solar tides are due to action of the sun, but 

 all risings and fallings of the water due to the action 

 of the sun are net tides. We want the quantifica- 

 tion of the predicate here very badly. We have a 

 true tide depending on the sun, the mean solar 

 diurnal tide, having for its period twenty-four solar 

 hours, which is inextricably mixed up with those 

 meteorological tides that I have just been speaking 

 of tides depending on the sun's heat, and on the 

 variation of the direction of the wind, and on the 

 variation of barometric pressure according to the 

 time of day. The consequence is that in tidal 

 analysis, when we come to the solar tides, we can- 

 not know how much of the analysed result is due 



