CHAP. IX.] TIDES 247 



the lower low water ; the tide then rises to the lower high 

 water, then falls to the higher low water, and finally completes 

 its round by a rise to the higher liigh water again. At others 

 this succession is reversed, but for most places the succession, 

 whichever way it takes place, is the same throughout the year. 

 Fig. 63 will show the two movements. 



When there is great but regular inequality, the higher tide 

 will always be in the daytime when the sun is on one side of 

 the Equator, and in the night-time when it is on the other. 



When the diurnal tides are great, the inequality in height 

 will sometimes be such as to cause a mere stand in the tide 

 during either the rise or fall of one tide, giving the effect of 

 only one high and low water in the twenty-four hours. 



Fig 63. 



As a general rule, it may be stated that in temperate latitudes 

 the highest tides take place at the equinoxes, whereas in the 

 tropics these occur at the solstices. 



Curiously enough, and it has affected many of our notions 

 about tides, the tides about the British Isles are the most simple 

 that are anywhere found — that is to say, so far as the individual 

 movement of the tide at any one place is concerned. But 

 they are largely affected by a further complication, known as 

 " Interference." By this is meant the appearance of another 

 tidal wave, or perhaps more than one, which affects the height 

 and time of the resultant tide at any place. 



This is caused by either a tidal wave coming round from 

 an opposite direction, or by one reflected from another coast, 

 and it will be at once seen that if the crests of such tidal waves 

 coincide at any point with the crest of the primary wave, the 

 resultant tide will be higher, and if the crest of one reaches 



