530 THE POPULAR SCIENCE MONTHLY 



nodal line running northeasterly from the Lesser Antilles is not ob- 

 scured. Eef erring now to the cotidal chart of the Atlantic Ocean (Fig. 

 5), it will be seen that the mean range of tide, as shown by Arabic nu- 

 merals, is seven feet along the coast of Georgia, between two and three 

 feet for the Bahamas, one foot for the outer coast of Porto Eico, and 

 little or nothing some leagues farther eastward. Hence the observa- 

 tional proof of the existence of one end of this nodal line. 



When it is high water along the Atlantic Coast of the United 

 States, it should be low water between Brazil and western Africa; that 

 is, because the tidal hour is twelve for the former locality, it should be 

 six for the latter. Doubtless such is the case for the portion of the 

 tide depending upon the system now under consideration. But the 

 system previously considered, viz., that extending from Guiana and 

 Brazil to southern Greenland, gives eight for the tidal hour off the 

 Brazilian coast. This explains why the observed times of tide between 

 western Africa and Brazil fall between six and eight o'clock. 



All along the southeastern coast of Brazil, the tidal hour is six, 

 while west of Cape of Good Hope it is about twelve — but not exactly 

 twelve because this locality is influenced by a progressive wave due to 

 the existence of the Gulf of Guinea, 



Farther south, along the Antarctic Continent, the tidal hour is 

 doubtless six, but no observations are available for verifying this 

 conclusion. 



It may be noted here, and before going farther, that upon the 

 small chart of the world (Fig. 4), the unshaded water areas are such 

 that forces acting upon them, and them alone, can produce little tide 

 either in such areas themselves or in other parts of the oceans. In 

 other words, if they possess tides, these will depend upon the tides ex- 

 isting in such portions of the oceans as are comparatively well suited 

 for their production. Heavy lines upon the chart indicate outer bound- 

 aries of systems. If rigid walls were erected along the outer boundaries 

 of any particular system, the forces would incite tides of considerable 

 size in the waters bounded by the walls and the shore lines; the tides 

 of the system, if kept down by resistance, would nearly agree in their 

 times of occurrence with the tides actually existing. These hypothetical 

 bodies of water, together with such landward dependencies as have their 

 tides occurring simultaneously with those of the body proper and upon 

 which the forces act, are shaded by means of light parallel lines. In a 

 particular ocean, the systems are distinguished by the directions given 

 to the lines of shading. Double shading indicates an overlapping of 

 systems. 



The waters surrounding the British Isles have, as a rule, large tides 

 accompanied by swift tidal streams. They depend chiefly upon the rise 

 and fall of that part of the ocean situated to the southwestward of 

 these isles. 



