Maj. Barnard on the Dynamic Theory of the Tides. 351 
“It was found necessary, however, (Airy ‘Tides and Waves,”) in order 
tomake the application of mathematics practicable, to start with two 
suppositions, which are inapplicable to the state of the earth. These are: 
the earth is covered with water; and that the depth of this water is 
the same through the whole extent of any parallel of latitude.” 
If the actual configuration of the ocean’s bed is, as I have be- 
fore remarked, the very basis of a dynamic theory of the tides, 
na theory which is obliged to reject entirely this actual con- 
figuration, and instead of ocean beds of Limited areas, isolated from 
each other by dry land in those parallels where the tidal effects 
ire greatest, substitute an imaginary ocean covering the whole 
globe, and of the same depth following each parallel of latitude, 
the problem can be only a mathematical one of more or less 
interest, from which nothing of any practical value, as to the 
actual phenomena of the tides, can be expected. 
Such 7s, in fact, the dynamic theory of Laplace; it has fur- 
tished no result nor been of the slightest use to physicists in 
their Investigations of the tidal phenomena. Mr. Air remarks, 
under thése suppositions (the arbitrary assumptions as to the 
peean's extent and depth) it is evident that the theory is far from 
being one of practical application;” but when we consider that, 
m the very effort to make the theory a dynamic one, by intro- | 
ducing the motions of the fluid particles, the real motions as gov- 
émed by the actual configuration of the ocean beds are discarded 
and purely Imaginary ones substituted, we may well hesitate in 
cme assent to the proposition which finishes the sentence; 
lene It clearly approaches much nearer to truth than the 
ny, of equilibrium which we have already described.” 
nthe eye of the mere theorist it may be so, but to one who 
eeKs a, knowledge of the tides as they actually are, the equilib- 
rca 1s far more useful; and of two things neither of 
: possess any claims to be called true, one may be con- 
The as true or the other. 
iin differentia] equations which determine the elevation and 
“ouon of the water, when the question is limited by these arbi- 
ty eo already mentioned, are obtained with no great 
the Wa 
dae Water itself are disregarded. We have only:to introduce 
and hte to convert the equilibrium into a dynamic cre A 
forces oe Considering the effects of the fluid motions only in the 
of a developed, it follows from the general equations 
vill : nar of fluids, that the total fluid pressure resulting 
Ach class Sum of the pressures due to the separate existence of 
of forces, 
