Cotidal Lines for the World 



3°7 



In straits or channels are amphidromic 

 regions caused by the deflecting force of 

 the earth's rotation acting upon a sta- 

 tionary oscillation which otherwise would 

 possess a nodal line. Examples of this 

 are the North Channel, the arm of the 

 North Sea between Holland and Eng- 

 land, Norton Sound, and the Strait of 

 Korea. 



There are still other amphidromic re- 

 gions due to other causes. 



It has been supposed that the tides of 

 the ocean advance westward around the 

 globe, endeavoring to follow the moon in 

 her apparent diurnal course in the 

 heavens. A westerly progression was 

 especially looked for in the southern 

 seas, where a continuous zone of water 

 encircles the earth. But what have we in 

 reality? A remarkable eastward pro- 

 gression in the Pacific Ocean due to the 

 opening between Cape Horn and Graham 

 Land forming a break in the rigid bound- 

 ary which constitutes the eastern support 

 of the South Pacific oscillating system. 

 The tide thus derived extends into the 

 Atlantic as far north as Uruguay. By 

 going eastward along the outer coast of 

 Antarctic Continent from io° east longi- 

 tude to within 45 ° of the starting point, 

 it is probable that whatever progression 

 exists is easterly and not westerly, but no 

 observations for the Antarctic Continent 

 are available. 



The tides in the Arctic Ocean proceed 

 easterly from Greenland Sea to Bennett 

 Island, Alaska, and northwestern British 

 America. 



The progression is easterly in the east- 

 ern half of the Indian Ocean. 



Excepting off the northern coast of 

 South America and off the southeastern 

 coast of the United States, the progres- 

 sion in the Atlantic is generally to the 

 north. This is due to the large openings 

 in the far north through which tidal ac- 

 tion is transmitted, to the particular trend 

 of the continental coast lines, especially 

 that of the western coast of Africa, and 

 to the continuity of the Antarctic Con- 

 tinent. 



The general progression in the North 

 Pacific is westerly, due largely to there 

 being many wave-producing openings 

 located on the western side of the ocean,, 

 while none are located on the eastern 

 side. 



In small deep bodies of water the tide 

 obeys the equilibrium theory, i. e., the 

 surface of the water remains normal to 

 the direction of instantaneous gravity. 

 Examples of this are Lake Superior, the 

 eastern portion of the Mediterranean 

 Sea (page 306), the southwestern corner 

 of the Gulf of Mexico, and of the Carib- 

 bean Sea. 



RANGE OF TIDES 



A few examples will now be given for 

 the purpose of showing that, unless ob- 

 scured by other effects, relatively large 

 ranges occur near the loops of the oscil- 

 lations and relatively small ones near the 

 nodal lines. 



The range of tide between Ceylon and 

 western Sumatra is probably less than 1 

 foot, while off the mouths of the Ganges, 

 the range is about 5 feet. 



At the northern end of Mozambique- 

 Channel the range is 9 feet, while around 

 Ras Hafun, Somaliland, the range is 

 about 3 feet ; going thence northerly, the 

 range again increases, being 5.6 feet at 

 Karachi and 8.8 feet at Bombay. 



The range of tide at Savannah River 

 entrance is 6.8 feet, while on the south 

 side of St Thomas Island it is but 0.3, 

 foot. 



The range of tide at Panama is 12.6 

 feet, while at Acapulco it is but 1.2 feet. 



(It has already been noted that the 

 tide of southwestern corner of the Carib- 

 bean Sea obeys the equilibrium theory. 

 The observed range at Colon is 0.6 foot.) 



On the eastern coast of the Philippine 

 Islands the range of tide is 4 feet, while 

 at the Island of Guam it is \]/ 2 feet. 



In many land-locked arms of the sea 

 the difference in range at loop and node 

 is still more apparent. Examples of this 

 are the Gulf of Suez, the English Chan- 

 nel, and the Irish Channel. I shall con- 



