Cotidal Lines for the World 



3°5 



Systems of Semi-diurnal Tide 



The overlapping of systems obscures 

 the loop around Cape Guardafui and the 

 loop along the coast of southern and 

 Lower California. 



The proximity of a loop or region to a 

 loop or region having a different tidal 

 hour necessitates a somewhat gradual 

 transition of the tidal hour as we pass 

 from the one locality to the other. This 

 tendency is quite universal, because in 

 the irregularly shaped bodies of water 

 found in nature it would be unreasonable 

 to expect to find extended nodal lines 

 and much less regions of any consider- 

 able extent having no tide at all. 



Several good approximations to nodal 

 lines are noticeable through the crowding 

 together of the cotidal lines : One ex- 

 tends from Ceylon to Sumatra ; another 

 extends northeasterly from the Wind- 

 ward Islands ; another southwesterly 

 from near Acapulco ; another southeast- 

 erly from Japan. 



Upon referring to the chart of cotidal 

 lines, it will be noticed that there are 

 several points from which the cotidal 

 lines for all hours seem to radiate, and so 

 must be points where the range of tide is 

 zero. These points and radiating lines 



are caused by the overlapping of systems, 

 by progressions due to secondary or de- 

 pendent bodies of water into which a free 

 wave progresses, and by the necessity of 

 a gradual change between adjacent re- 

 gions whose tides are not simultaneous. 



The existence of such a point can be 

 readily seen in the case of a square area 

 having two stationary waves at right 

 angles to each other, but differing in 

 phase by some amount other than o° or 

 180 . The no-tide point will be located 

 at the intersection of the two nodal lines. 

 Around this point the tidal hours will 

 progress, completing a cycle of values in 

 the period of the oscillation. Hence the 

 radiating cotidal lines. 



One, two, or three of the causes alluded 

 to may be instrumental in bringing about 

 a no-tide point around which the cotidal 

 lines may be styled "amphidromic." 



The no-tide points and amphidromic 

 regions in mid-ocean are located as fol- 

 lows : Between Madgascar and southern 

 Hindostan ; westerly from the Azores ; 

 between California and the Hawaiian 

 Islands ; northwesterly from the Society 

 Islands, and southeasterly from New 

 Zealand. 



