374 PHYSIOGRAPHY. [chap. 



height of high water and a trough the depth of low water, 

 had passed twice round the parallel in the same space of 

 time. 



Thus, the rotation of the earth, combined with the attrac- 

 tion of the ocean by the sun and moon, gives rise to solar 

 and lunar tidal waves. If the free motion of the waters 

 of the ocean were not interfered with by the conformation 

 of the land, and if there were no moon, high water would 

 always take place a little after noon and midnight ; and low 

 water would be a little after six o'clock in the morning and 

 evening. Moreover, the rise and fall of these solar tides 

 would be much less than our actual tides. For the great 

 distance of the sun weakens his tide-producing value to such 

 an extent that his effect, as compared with the moon's, is 

 only as 4 to 9, or thereabouts. 



The lunar tides, therefore, are much more important than 

 those caused by the sun. If the moon always came to the 

 meridian at the same time as the sun (as is the case at new 

 moon) it is obvious that the lunar tide would strengthen the 

 solar tide, and solar and lunar high waters and low waters 

 would correspond. 



Again, if the moon were always 180° from the sun (as is 

 the case at full moon) the attractions of both would still 

 conspire, though not so completely, to the same end, and the 

 times of high and low water of both would still coincide. 



On the other hand, if the moon always came to the meri- 

 dian six hours sooner or later than the sun, it is obvious 

 that the two tidal waves would tend to neutralize one 

 another. It would be sun low-water when it was moon 

 high-water, and vice versa. In the former cases the high or 

 low water would be the sum (or nearly so) of the solar and 

 the lunar high or low waters, while, in the latter, it would 

 be their difference. 



