82 



On the supposed Tides in the 



Fig. 2. " The surface of the sphere being supposed to be 

 flattened, and the tides spread on it, they would assume the 

 form of the waves here shown. The dotted straight hne 

 shows the mean height, which is a Httle above the surface in 

 the principal sections of the spheroid, although not uni- 

 versally." 



J)^.3 



Fig. 3. " The nature of the tides of lakes, the surface be- 

 ing regulated by that of the dotted Hne in Fig. 2. nearly 

 agreeing with it in direction, as at D, when the lake is nar- 

 row and deep ; but differing from it, as at E, when shallow." 

 — Yoicng''s Natural Philosophy, Vol. I. p. 793. 



The area and depth of a lake being known, Doct. Young 

 has given a theorem, in the second volume, of his Lectures, 

 page 343, by which the maximum rise and fall of the water, 

 and the time of each oscillation, or in which a tide-wave 

 might pass over it, can be ascertained.. 



The same causes may operate to elevate the tide in nar- 

 row parts of lakes, above the level of that, theoretically de- 

 duced for, or actually indicated in, their most expanded por- 

 tions, as in the gulfs, bays, straits and mouths of rivers con- 

 nected with the ocean ; and it may also be increased, or di- 

 minished, by the effect of the winds. Thus a very small 

 tide, of only a few inches, on the margins of the lake, at the 

 points of its greatest breadth and profundity, may be swell- 

 ed into one of some feet, in the narrbw channels of estuaries, 

 and the prolonged indentations of the coast ; for although 

 '■'■ the primitive tide''"' is only five feet, and upon the shores of 

 the broad and deep ocean rarely exceeding, from extraneous 

 causes, ten ; still, when it is impeded in its course, or enters 

 gulfs, which plunge far into the land, with diminishing ex- 

 tremities, it rises to the height of forty, fifty and even an hun- 



