3°4 



The National Geographic Magazine 



high water along the coast of the United 

 States, is it low water at a distance of y 2 



(i. e., half a wave length) to the south- 

 east? For a depth of 3,000 fathoms y 2 A 

 is, by computation, 46.6 degrees of a 

 great circle. This carries one from the 

 American coast to a point southwesterly 

 of the Cape Verde Islands ; and here it 

 will be seen the lines indicate that the 

 tidal hour is approximately VI. But they 

 indicate more, namely, that there is a 

 region of considerable size over which 

 the time of tide changes by only a small 

 amount. The lateral boundaries consist 

 of the northeastern coast of eastern 

 Brazil, the African coast from Liberia to 

 Bijouga Islands and the Cape Verde 

 Islands. 



Having shown the existence of a sta- 

 tionary wave in the ocean, we may now 

 briefly consider what systems of station- 

 ary waves or oscillations are possible in 

 the various oceans. Without going into 

 details, it may be said that the regions or 

 "areas" considered must have depths and 

 horizontal dimensions such that their free 

 periods of oscillation approach 12 lunar 

 hours, or the period of the tidal forces by 

 Which the motions are sustained. 



The determination of the free period 

 and mode of oscillation of a body of 

 water is usually a difficult mathematical 

 problem, even when the depth is uniform, 

 the boundaries are simple, and no ac- 

 count is taken of the deflecting force of 

 the earth's rotation. If some of the 

 boundaries are wanting, the oscillation is 

 somewhat imperfect in its character and 

 has not been investigated by mathema- 

 ticians or physicists. A few experiments 

 will, however, convince one that he can 

 often obtain through the consideration of 

 simple bodies an approximation to the 

 periods of bodies having variable depths 

 and having imperfect as well • as rather 

 complicated boundaries. 



The systems or combination of "areas" 

 possessing a period of approximately 12 

 lunar hours are shown on an accompany- 

 ing chart (page 305) and may be briefly 

 described as follows : 



The North Indian system covers the 



more land-locked portion of the Indian 

 Ocean. The South Indian system -ex- 

 tends from the south coast of Australia 

 southwesterly to the Antarctic Continent 

 and thence northwesterly to Madagascar 

 and South Africa. The South Atlantic 

 system extends from the Antarctic Con- 

 tinent north, partly to Madagascar and 

 South Africa and partly to the 27th 

 parallel of south latitude west of South 

 Africa. From this parallel one branch 

 extends west-northwest to the eastern 

 coast of Brazil, and another northwest- 

 erly to the Atlantic coast of the United 

 States. The North Atlantic system ex- 

 tends northeasterly from the northeastern 

 coast of Brazil to points west of Morocco ; 

 thence northwesterly to Greenland and 

 Labrador. The South Pacific system 

 comprises a belt extending from southern 

 Chile and Graham Land westerly and 

 northwesterly to the islands and shoals 

 north of New Zealand ; thence northeast- 

 erly to the coast of southern and Lower 

 California. The North Pacific, system 

 extends over nearly all of the North Pa- 

 cific Ocean and also covers a broad band 

 extending from Chile westerly to and 

 joining the North Pacific Ocean. 



The tidal forces acting upon these sys- 

 tems cause the tides to occur at times 

 which can be approximately determined 

 a priori. Such times are written upon 

 the loops of the systems, and in all cases 

 approximately agree with the observed 

 times of tide for these localities. 



Upon referring to the chart of cotidal 

 lines, it will be noticed that some of the 

 loops of the systems are more conspicu- 

 ous than others. For instance, the time 

 of tide is nearly simultaneous over a con- 

 siderable region at each of the follow- 

 ing loops : East of Brazil, off Sierra 

 Leone, off the Atlantic coast of the 

 United States, off Gibraltar, off Panama, 

 and off the Philippine Islands. Extensive 

 progressions conceal in great measure 

 the stationary waves occurring in the 

 following localities: Between Java and 

 northwestern Australia, near Cape Good 

 Hope, near Cape Farewell, and along the 

 coast of Chile. 



