292 KELVIN 



they think nothing of the double low water. Hence we hear 

 continually of very high tides very high water noticed at 

 the time of high tides when the barometer is low. We 

 have not always, however, in this effect of barometric 

 pressure really great tidal rise and fall. On the contrary 

 we have the curious phenomenon that sometimes when the 

 barometer is very low, and there are gales in the neighbour- 

 hood, there is very little rise and fall, 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 not tides. We 

 want the quantification 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 analy- 

 < sis, when we come to the solar tides, we cannot know how 

 j much of the analysed result is due to attraction, and how 

 1 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 gravi- 

 tational, 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 com- 

 bination of theory and observation : if these afford sufficient 

 reason for believing that the motion is due to attraction of 



