president's address. 625 



arise from one or other of different causes, but we have in such 

 a case merely to do with two different wave-systems acting 

 simultaneously. Each series has its own properties, and the 

 larger waves arrive at their proper intervals quite independently 

 of the smaller. 



Next to waves quite the most striking physical phenomenon 

 connected with the sea is that of tides. A tide is by no means 

 the simple movement of the water which might be supposed. 

 There are many complicating influences besides the ever-varying 

 incidences of the attraction of moon and sun, and the tide is 

 really a summation of all these. I will not, however, deal par- 

 ticularly with the problem of tidal analysis, but I desire more 

 especially to speak of one of the more important results of the 

 action of tides upon the fate of the world. The effects of the tides 

 on terrestrial motion are of the most profound importance, for 

 the result of their action is, through friction, to slow down the rate 

 of rotation of the earth, and thus to lengthen our day. So great, 

 however, is the energy by virtue of which the earth is spinning 

 on its axis, that the enormous force with which the tides tend to 

 retard it is only competent in 1,000 years to increase the length of 

 our day by a small fraction of a second. As Sir Robert Ball has 

 very truly said, however, " what may be a very small matter in 

 one thousand years can become a very large one in many millions 

 of years." This will help us in our attempts to form some mental 

 conception of the expanse of time which is involved in the 

 history of the world. There was a time when our earth whirled 

 round its axis in some three hours, that being the length of the 

 day, and we calculate that it was somewhere about this period 

 that the moon was thrown off. Since that unspeakably remote 

 period the moon has been steadily at work acting as a break on 

 the world, and at the same time has been continually increasing 

 her own distance from us. In obedience to a well defined law, 

 the moon is ceaselessly withdrawing from the earth, her orbit 

 being really an ever expanding spiral. This is a n< cessary result 

 of tidal action, and the distance of the moon from us is rigidly 

 determined by tlie rate at which the world revolves. Let us 



