GENERAL CHARACTERISTICS 



11 



arrow (fig. 8), and the tide high at the place marked A 1 , just under 

 the inoon, then, in about six hours, this place will have been carried 

 round to A 2 , where the tide is low ; and, after similar intervals, to 

 A 3 and A 4 successively, where the tide is high and low respectively. 

 Hence the daily changes are to a great extent determined by the 

 rotation of the earth. 



But we have already observed that each change of tide occupies 

 a little more than six hours, the average time being nearly six 

 hours and a quarter, and so we find that the high and low tides 

 occur nearly an hour later every day. This is due to the fact that, 

 owing to the revolution of the moon round the earth in the same 

 direction as that of the rotation of the earth itself, the day as 



FIG. 8. ILLUSTRATING THE TIDES 



measured by the moon is nearly an hour longer than the average 

 solar day as given by the clock. 



There is yet another point worth noting with regard to the 

 relation between the moon and the tidal movements of the water, 

 which is that the high tides are never exactly under the moon, but 

 always occur some time after the moon has passed the meridian. 

 This is due to the inertia of the ocean, and to the resistance offered 

 by the land to its movements. 



Now, in addition to these diurnal changes of the tide, there are 

 others, extending over longer periods, and which must be more or 

 less familiar to everyone who has spent some time on the coast. 

 On a certain day, for instance, we observe that the high tide flows 

 very far up the beach, and that this is followed, a few hours later, 

 by an unusually low ebb, exposing rocks or sand-banks that are 



