412 NEWTON S PRINCIPIA. 



forced wave following the moon. On reaching the Cape of 

 Good Hope, one tide wave is sent up the Atlantic north 

 wards, and another continues its course into the Pacific, 

 and travels westwards and northwards up the west coast 

 of America in a very remarkable manner. The tide wave 

 in the Atlantic travels, therefore, from the south north 

 wards, and arrives at England fourteen hours after it left 

 the Cape. It here sends branches into the English and 

 other channels that environ England, while the chief wave 

 continues its progress northwards, sending other tide waves, 

 travelling from north to south, down the same channels. 

 Two tide waves arrive, therefore, at the mouth of the 

 Thames, and the London tide is compounded of these two. 

 One has travelled along the south coast in seven hours, 

 and the other has gone all round Scotland and descended 

 the German Ocean in about twenty hours. The difference 

 of these intervals is about twelve hours; thus the two 

 semi-diurnal waves arrive in nearly the same phase and 

 strengthen each other, and the two diurnal waves in oppo 

 site phases and destroy each other. There is therefore at 

 London no diurnal variation of the tide. These two 

 tides, though they meet at the Thames, do not travel in 

 exactly opposite directions. They both travel a little 

 eastwards, in direct opposition to the motion of the moon. 

 They are both also greatly modified by the many shoals 

 found in those seas. As may be supposed there are points 

 where the two tides meeting in opposite phases, even the 

 semi-diurnal tide will be destroyed. Captain Hewitt has 

 discovered such a point. It is situated about latitude 

 52J ; there is simply an alternate tidal current, but no 

 elevation of the water. 



The tide wave in the Atlantic is probably partly formed 

 in the Atlantic and partly derived from the South Seas. If 

 it were entirely formed in the Atlantic, as the length of 

 that sea from south to north is greater than its breadth, 

 the tide wave would have a tendency to travel partly along 

 its length, with a very irregular velocity, and partly along 

 its breadth. If the tide were entirely derived from the 



