ON THE TIDES. 445 



is perhaps 12 or 13 hours in its passage to the coast of France, but cer- 

 tainly not more. This tide, which would happen at the sixth lunar hour 

 after the moon's transit, if there were no resistance, is probably so checked 

 by the resistance, that the water begins to subside about the fourth, and in 

 some seas even somewhat earlier, although in others it may follow more 

 nearly its natural course. There is scarcely a single instance w r hich 

 favours the supposition of the time of high water in the open sea being 

 within an hour of the moon's southing, as it must be if the depth were 

 very great ; so that neither the height of the tides nor the time of high 

 water will allow us to suppose the sea any where quite so deep as 4 

 miles. 



The tide entering the Atlantic appears to advance northwards at the rate 

 of about 500 miles an hour, corresponding to a depth of about 3 miles, so 

 as to reach Sierra Leone at the 8th hour after the moon's southing ; this 

 part of Africa being not very remote from the meridian of the middle of 

 the south Atlantic ocean, and having little share in the primitive tides of 

 that ocean. The southern tide seems then to pass by Cape Blanco and 

 Cape Bojador, to arrive at Gibraltar at the 13th hour, and to unite its 

 effects with those of other tides at various parts of the coast of Europe. 



We may therefore consider the Atlantic as a detached sea about 3500 

 miles broad and 3 miles deep ; and a sea of these dimensions is susceptible 

 of tides considerably larger than those of the ocean, but how much larger 

 we cannot determine without more accurate measures. These tides would 

 happen on the European coasts, if there were no resistance, a little less 

 than five hours after the moon's southing, and on the coast of America, a 

 little more that seven hours after ; but the resistance opposed to the motion 

 of the sea may easily accelerate the time of high water in both cases about 

 two hours, so that it may be a little before the third hour on the western 

 coasts of Europe and of Africa, and before the fifth on the most exposed 

 parts of the eastern coast of America ; and in the whole of the Atlantic, 

 this tide may be combined more or less both with the general southern 

 tide, and with the partial effects of local elevations or depressions of the 

 bottom of the sea, which may cause irregularities of various kinds. The 

 southern tide is, however, probably less considerable than has sometimes 

 been supposed, for, in the latitudes in which it must originate, the extent 

 of the elevation can only be half as great as at the equator ; and the Islands 

 of Kergulen's Land and South Georgia, in the latitudes of about 50 and 

 55, have their tides delayed till the 10th and llth hours, apparently 

 because they receive them principally from distant parts of the ocean, 

 which are nearer to the equator. 



On the western coasts of Europe, from Ireland to Cadiz, on those of 

 Africa, from Cape Coast to the Cape of Good Hope, and on the coast 

 of America, from California to the streights of Magellan, as well as in the 

 neighbouring islands, it is usually high water at some time between two 

 and four hours after the moon's southing ; on the eastern coast of South 

 , America between four and six, on that of North America between seven 

 and eleven ; and on the eastern coasts of Asia and New Holland between 

 four and eight. The Society islands are perhaps too near the middle of the 



