172 Baron N. Schilling on the Constant Currents 



This circumstance seems at the first glance to contradict the 

 theory of the moon's attraction; yet the origination of this 

 equatorial streamless zone is easily explained when we reflect 

 that the moon and sun are simultaneously in the vicinity of the 

 equator only for a very brief time twice yearly. They usually 

 describe parallel circles which lie -between the equator and the 

 tropics ; the moon only goes sometimes slightly beyond the last- 

 mentioned circles. The ellipsoid arising from the united attrac- 

 tions of the moon and sun must always have its cusps between 

 the sun and moon ; and hence these cusps must mostly describe 

 parallels between the equator and the tropics. 



Supposing the tidal ellipsoid in the position asep (fig. 6), 



Fig. 6. 



its major axis a e making a certain angle with the equator 

 ACED ; by the earth's rotation on its axis P S the cusps a 

 and e of the ellipsoid will describe the parallel circles a a! and 

 ee ! ; and therefore the maximum of the flood- current will also 

 be observed on these parallel circles. The current will also not 

 preserve its exact east- to- west direction, but, as shown by the 

 arrows B andF, come from E.S.E. in the southern hemisphere, 

 and from E.N.E. in the northern. 



On both sides of the parallel circles a a' and e e 1 the force of 

 the flood-current will, as already said, diminish rapidly; while 

 the ebb-current keeps, as before, the maximum of its force at 

 the greatest circle, therefore at the equator, and, also with this 

 position of the ellipsoid, diminishes only slowly polewards, con- 



