‘ Astronomical and Nautical Collections. 307 
has calculated, that in an ocean of équable depth, the difference 
between the heights of the morning and evening tides, depending 
on the declination of the luminary, must wholly disappear ; but 
we cannot help suspecting that there must be an imperfection in 
some of the many steps of his investigation. The depth would be 
equable if the whole sphere were fluid; and it will not be denied 
that in this case there would be a difference in the morning and 
evening tides, very nearly coinciding with that of the primitive 
variations of the figure affording an equilibrium: nor can we dis- 
cover any imperfection in the method, which Mr. Laplace himself 
has sometimes adopted, of considering the difference of the two 
tides as a separate diurnal tide, and determining its magnitude pre- 
cisely in the same manner as if it existed alone.” 
“When a regular tide moves continually forwards in an open 
ocean, the progressive motion of the fluid is the greatest, or in other 
words, the flood is the strongest where the elevation is greatest, and 
the motion is retrograde, constituting the ebb, wherever there is a 
depression. In a river, the effect of a stream would only so far 
modify the velocity, as to make it proportional to the elevation 
above or the depression below a different level; but if a river or 
channel of any kind terminated abruptly, so as to cause a reflec- 
tion, the progressive velocity would commence from the time of low 
water, and continue till that of high water only, or even be coun- 
teracted by the motion of the current, so as to cease still earlier, 
and to commence later. The rivers, in which our tides are com- 
tionly observed, seem to hold a middle place between these two 
¢asés : at Lambeth, for instance, the flow of the tide is continued, 
not during the whole time that the watet remains elevated above a 
cettain level, but about three quarters of an hour after the time of 
high water, at which it would cease near the end of a channel ter- 
minating abruptly. And it is probable that by similar considera- 
tions the course of the ebb and flood tides might be explained in 
many other cases.” 
“If we apply the same mode of calculation to the tides of the 
atmosphere, they will appear to be subject to some very singular 
modifications, At the poles they must be very small; at the equa 
