THE TIDES. (APP. C) 205 



spring tides and neap tides due to the alternately 

 conspiring and opposing actions of the moon and 

 sun, and with large irregular variations produced 

 by wind. Careful observation detects a small 

 " diurnal " inequality (so called because it is due 

 to tidal constituents having periods approximately 

 equal to twenty-four hours lunar or solar), of which 

 the most obvious manifestation is a difference at 

 certain times of the month and of the year 

 between the heights of the two high-waters of the 

 twenty-four hours, and at intermediate times a 

 difference between the heights of the two low- 

 waters. 



In the western part of the North Atlantic and in 

 the North Sea, this diurnal inequality is so small in 

 comparison with the familiar twelve-hourly or 

 "semi-diurnal" tide that it is practically disre- 

 garded, and its very existence is scarcely a part of 

 practical knowledge of the subject ; but it is not so 

 in other seas. There is probably no other great 

 area of sea throughout which the diurnal tides are 

 practically imperceptible and the semi-diurnal tides 

 alone practically perceptible. In some places in 

 the Pacific and in the China Sea it has long been 

 remarked that there is but one high water in the 

 twenty-four hours at certain times of the month, 

 and in the Pacific, the China Sea, the Indian Ocean, 

 the West Indies, and very generally wherever tides 

 are known at all practically, except on the ocean 

 coasts of Europe, they are known to be not 



