82 On the supposed Tides in the 
form of the waves here-shown. The dotted straight line 
shows the mean height, which is a little above the surface in 
the principal sections of the spheroid, although not uni- 
versally.”’ ' 
Fig. 3. “ The nature of the tides of lakes, the surface be- 
ing regulated by that of the dotted line 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.” 
—Young’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 narrow channels of estuaries, 
and the prolonged indentations of the coast; for altho 
“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- 
