116 THE OCEAN. 



ing a shorter or longer period, and then adding to the ebb cannot 

 but augment its duration.* A fact more difficult to explain is that, 

 whilst in the greater number of ports remote from any river's 

 mouth, the rising tide is shorter than the fulling, numerous instances 

 of the opposite are to be seen ; and especially the port of Holyhead. 

 According to the hypothesis generally adopted, this longer duration 

 of ebb ought to be attributed to the rotation of the earth in the direc- 

 tion of west to east. The tidal wave being propagated in the con- 

 trary direction, that is to say, from east to west, would meet a 

 certain resistance in the waters which are spread before it. It would 

 rise up and become steeper and more rapid towards the west ; while 

 its other slope, that of the ebb, would lengthen itself towards the 

 east. This will explain why the phase of the flow does not last so 

 long as that of the ebb. 



The inequalities, which are observed in certain parts between two 

 successive tides, are likewise a strange and, in some respects, unex- 

 plained phenomenon. These various inequalities, now in the dura- 

 tion, and now in the respective lieights of the two tides of morning 

 and evening ; or which even affect everj^ oscillation in its entire 

 course ; arise in part from the declination of the moon, that is to say, 

 from its varying distance to the south or north of the equinoctial line. 

 I3ut in many cases the differences between two successive tides are 

 relatively enormous, and this explanation is not sufficient. Thus at 

 Port Essington, on the northern coast of Australia, differences in 

 height of nearly 4 feet between the oscillation of evening and morn- 

 ing have been observed. At Singapore, where the mean tide during 

 the time of highest water is nearly 7 feet, the difference between 

 two succeeding tides is sometimes nearly 5 feet. At Kurrachee the 

 daily variation is no less, and in the Gulf of Cambay it attains to 

 nearly 7 feet. At Bassadore, at the entrance of the Persian Gulf, the 

 duration of one oscillation of the sea sometimes exceeds by two hours 

 that which follows it ; and, finally, it has happened at Petropau- 

 lowski, in the northern Pacific, that expected tides have never appeared 

 at all. We can explain these singular anomalies only by the inter- 

 section of several reflex waves, diurnal and semidiurnal, which inter- 

 fere with one another ; and the confused oscillations of which are 

 produced by the meeting of moving liquid masses of diverse origin. 

 It is thus that on the surface of a pond, the waves that have risen at 

 different points form an immense network of intersecting lines, which 

 the breeze mingles in undecided wavelets. 



* See below, p. 123. 



