THE TIDES 121 



the case of the figure is for the flood-tide from 3 hours 

 6 minutes before to 3 hours 6 minutes after high-water. On 

 the contrary, the ebb runs so long as the particle lies under 

 mid-water level that is, from 3 hours 6 minutes before low- 

 water to 3 hours 6 minutes after low-water. It is slack water 

 at the moment the particle passes through the mid-water 

 position. 



This phenomenon in deep water appears quite different if 

 one stands on the coast. Here the change in the direction of 

 the current does not follow 3 hours 6 minutes after high or low 

 water, but coincides with it. 



This change in the tidal stream does not, however, take 

 place everywhere at high and low water. On London Bridge 

 the flood-stream continues to run even after the water has 

 fallen 2 feet, as can easily be noted by direct observation. 



Seamen know well that in the middle of the English 

 Channel the flood runs three hours after high-water and the 

 ebb three hours after low-water that is, exactly according to 

 the above theory. In the Pentland Firth, between Stroma and 

 Swona, the stream changes exactly three hours after high and 

 low water. 



Our knowledge of the tides and tidal streams of the world 

 is still very incomplete. There is a complete lack of tidal 

 observations in the centres of the great oceans. 



Only in the southern ocean is there a complete water-belt 

 round which it is possible for the two tidal waves to travel. 

 These are known as primary waves. 



A primary wave sweeping round the southern ocean, 

 passing in succession the southern coasts of Australia, Africa, 

 and South America, may be assumed to give off secondary 

 waves which pass up the three great oceans in a more or less 

 northerly direction. These are derived waves, and from them 

 arise the tides along the various coasts which they pass. 

 Consider the Atlantic Ocean. 



On this theory a derived wave passes up it. It is deflected 

 and retarded by the continental coasts, as it passes from south 



