22 Of TID'ES. 



lunar hour. It sends a wave into the Atlantic, which b perhaps 

 twelve or thirteen hours in its passage to the coast of France, but 

 certainly not more. This tide, which would happen at the sixtlv 

 kiuar 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 which 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 four 

 miles. 



The tide entering the Atlantic appears to advance northwards at 

 the rate of about five hundred miles an hour, corresponding to a 

 depth of about three miles, so as to reach Sierra Leone at the 

 eight hour after the moon's southing; this part of Africa being not 

 very remote from tin? meridian of the middle of the south Atlantic 

 ocean, and having little share in the primitive tides of that ocean. 



The southern tide seems ^heu to pass by Cape Blanco and Cape 

 JBojador, to arrive at Gibraltar at the thirteenth 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 

 350O miles broad and three miles deep ; and a sea of these dimen- 

 sions 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 kss than five hours after 

 the moon's southing, and on the coast of America, a little more 

 than 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,. 



