CH.II] RHYTHMICAL CHANGE IN THE SEA 29 



those which are repeated with the greatest regularity. 

 Organic changes depend on these physical ones, but 

 the inter-relationship is so complex that it is some- 

 times difficult to detect the periodicity. 



About twice in every twenty-four hours the tide 

 encroaches on the land, converting what a few hours 

 ago were desolate expanses of sand and mud into 

 evanescent sheets of water ; and again laying them 

 bare to the atmosphere. Observing the order of the 

 tides a little more closely we notice that they do 

 not recur at precisely the same time, nor do they 

 rise to exactly the same height on two successive 

 days. Shortly after the time of full moon the water 

 rises to its maximum, and then for about a week the 

 flood tide culminates a little later every day and 

 rises a little less high. About halfway between the 

 times of full moon and new moon the velocity of the 

 tidal stream is least, and so also is the height to 

 which the water rises. Then the tides begin to 

 gather force and shortly after the date of new moon 

 they again attain a maximum. Thus there is a 

 half-daily rhythm, for a rise and fall of the tides 

 occurs twice during each twenty-four hours ; but 

 there is also a fortnightly rhythm, for spring tides 

 and neap tides occur twice in each lunar month. 

 If we continue to observe these things throughout an 

 entire year we find also that every six months, about 

 the times of the vernal and autumnal equinoxes, 



