THE TIDES 291 



direction of the wind say in the direction of the two 

 arrows near to the surface of the water above and be- 

 low it. But to re-establish the disturbed hydrostatic 

 equilibrium, the water so heaped up will tend to flow 

 back to the end from which it has been displaced, and as 

 the wind prevents this taking place by a surface current, 

 there will be set up a return current along the bottom of 

 the canal, in a direction opposite to that of the winds, as indi- 

 cated by the lowermost arrow in the diagram (FiG. 123). 

 The return current in the ocean, however, is not always an 

 under current, such as I have indicated in the diagram, 

 but may sometimes be a lateral current. Thus a gale of 

 wind blowing over ten degrees of latitude will cause a drag 

 of water at the surface, but the return current may be not 

 an under current but a current on one side or the other 

 of the area affected by the wind. Suppose, for instance, 

 in the Mediterranean there is a strong east wind blowing 

 along the African coast, the result will be a current from 

 east to west along that coast, and return current along 

 the northern coasts of the Mediterranean. 



The rise and fall of the water due to these motions are 

 almost inextricably mixed up with the true tidal rise and 

 fall. 



There is another rise and fall, also connected with the 

 heating effect of the sun, that I do not call a true tide, and 

 that is a rise and fall due to change of atmospheric pres- 

 sure. When the barometer is high over a large area of 

 ocean, then, there and in neighboring places, the tendency 

 to hydrostatic equilibrium causes the surface of the water 

 to be lower, where it is pushed down by the greater weight 

 of air, and to be higher where there is less weight over it 

 It does not follow that in every case it is lower, because 

 there may not be time to produce the effect, but there is this 

 tendency. It is very well known that two or three days of 

 low barometer make higher tides on our coast. In Scot- 

 land and England and Ireland, two or three days of low 

 barometer generally produce all round the shore higher 

 water than when the barometer is high; and this effect is 

 chiefly noticed at the time of tidal high water, because 

 people take less notice of low water as at Portland where 



