in the Air and in the Sea. 



167 



above the ordinary sea-level as the ebb sinks below it. Secondly, 

 it is assumed that the middle time between high and low water 

 corresponds to the normal level. The highest water is formed 

 by the two cusps A and E (fig. 3) of the tidal ellipsoid APES, 



Fig. 3. 



and the lowest by the circle P S, which halves the surface of the 

 ellipsoid at its minor axis. The normal level will therefore, ac- 

 cording to the common assumption, be found on the circles D C 

 and F G, which run parallel with the circle P S and are distant 

 45° of arc both from the points A and E and from the circle 

 PS; so that PF = FE and PC = AC; that is, about three 

 hours after flood the normal level, and three hours later the ebb 

 comes in. On this assumption, however, the superficial space 

 of the surfaces ACD and EFG together, occupied by the flood 

 tide, is 2 \ times as small as the superficies of the middle zone 

 C F GD, in which the water stands at the ordinary level. But 

 since the water which forms the accumulation of the flood can 

 only be derived from the ebb-zone, it is clear that, on this assump- 

 tion, the same mass of water must rise considerably more on the 

 smaller space than the water-surface sinks in the ebb-zone. If, 

 on the other hand, we adhere to the assumption that the water 

 rises as high above the normal level as it sinks below it, the sur- 

 face occupied by the two floods must be just as great as that 

 occupied by the middle ebb-zone, and the two circles at which 

 the normal level is found must be only 30° distant from the 

 central circle, but 60° from the cusps A and E of the ellipsoid. 

 Flood tide would, thus last eight hours, but ebb only four. Or 

 the w T ater must fall as much in the last two hours of its going 

 down as in the first four after high water, and likewise rise as 

 much in the first two hours after its lowest as in the remaining 

 four. Probably the reality lies between the two assumptions ; 

 that is, the rise of the water during flood is probably more con- 

 siderable than its fall during ebb, and, on the other hand, the 



