Great North American Lakes. 



81 



as depth of the lake. In the case of a direct tide^ the time 

 of the passage of the luminary over the meridian must coin- 

 cide with that of high water, and in the case of an inverted 

 tide, with that of low water. 



" In order that the height of the inverted or remote lunar 

 tides may be five feet, or equal to that of the primitive tides, 

 the depth of the open sea must be six and a half miles; and 

 and if the height is only two feet, which is perhaps not far 

 from the truth, the depth must be three and five-seventh 

 miles. 



" The tides of a lake, or narrow sea, differ, materially, 

 from those of the open ocean, since the height of the water 

 scarcely undergoes any variation, in the middle of the lake ; 

 it must always be high water at the eastern extremity, when 

 it is low water at the western : and this must happen at the 

 time, when the places of high and low water, with respect 

 to the primitive tides, are equally distant from the middle of 

 the lake. [Figs. 1. 2. and 3. from Plate 38.] 



" The tides may be direct, in a lake, one hundred fathoms 

 deep, and less than 8° wide ; but if it be much wider, they 

 must be inverted. 



" Hitherto we have considered the motion of the water as 

 free from all resistance ; but where the tides are direct, they 

 must be retarded by the effect of a resistance of any kind ; 

 and where they are inverted, they must be accelerated ; a 

 small resistance producing, in both cases, a considerable dif- 

 ference in the time of high water." — Young's Natural Phi- 

 losophy, Vol. I. p. 578. 



Fig. 1. 



Fig. 1. "The dotted ellip- 

 sis shows the section of a 

 spheroid, which would be 

 the form of the earth and 

 sea, if it were always in a 

 state of equilibrium, with the 

 attraction of a distant body ; 

 and the dark ellipsis, the ac- 

 tual form assumed, in conse- 

 quence of its rotation round 

 its centre, the depth of the 

 sea being less than thirteen 

 miles." 



Vol. XVI.— No. 1. 



11 



