818 
culty of treating the motions of fluids; thirdly, the 
peculiar difficulty of treating the motions when the 
fluids cover an area which is not plane but convex ; 
and, fourthly, the sagacity of perceiving that it was 
necessary to consider the earth as a revolving body, 
and the skill of correctly introducing this considera- 
tion. The last point alone, in our opinion, gives a 
greater claim for reputation than the boasted explana- 
tion of the long inequality of Jupiter and Saturn.” ! 
We must, however, qualify this eulogy by adding, in 
the words of the same writer, that Laplace’s theory, 
though based on sounder principles than the equili- 
brium theory, “ has far too little regard to the actual 
state of the earth to serve for the explanation of even 
the principal phenomena of the tides.” It is, in fact, 
like many other productions of the same age and 
school, a great display of ingenuity and mathemati- 
cal skill, which hardly yields a single result worthy 
of confidence, or agreeing with nature, except by the 
abandonment of its deductive rigour, or a concealed 
induction backwards from the phenomena to be ac- 
counted for. The same amount of skill and resource 
which Mr Airy has shown in adapting it to his own 
views, and to recent observations, would probably 
have sufficed to construct a theory from the founda- 
tion. By others the attempt seems to have been 
abandoned as hopeless. 
Ps 5 Since our limits will not permit us to return to 
Tides.Dr the subject of tides, we shall here briefly state the 
Whewell— progress of the subject since the time of Laplace, 
Sir J. Lub- The chief steps have consisted in co-ordinating the 
xis results of observation and analyzing them into their 
partial phenomena, by the help of Newton’s and 
Bernouilli’s theory, This labour has been greatly 
advanced by Dr Whewell, and also by Sir John Lub- 
Seng bock, The former has constructed maps of “ cotidal 
ines. 
lines,” which, indicating the relative time of high 
water in different parts of the globe, give us a gra- 
phic conception of the course and propagation of the 
tidal wave. The tides of the Hastern Pacific are but 
little known; but a vast wave advances northwards 
between Australia and Africa, diverted or retarded 
by the obstacles it meets with in the Indian Archi- 
pelago. Another (and to us the most important) 
branch sets from south to north up the vast canal of 
the Atlantic, where it is gradually complicated by 
local tides, having their origin in the wide expanse 
between Africa and the Gulf of Mexico, The two 
sets of waves sometimes reinforce, sometimes oppose, 
one another; they are prolonged to the western 
shores of England and Norway, where the tidal im- 
pulse arrives 24 hours after it passed the Cape of 
Good Hope. It is propagated most rapidly at a dis- 
tance from coasts, and is retarded in narrows and 
shallows. It sends offshoots into every bay and 
strait, always greatly retarded in point of time (ap- 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
parently by friction), but often increased in elevation 
by concentration of the effect in a gradually narrow- 
ing channel, as we see in the exaggerated tides of the 
river Amazon, the Severn, and the Bay of Fundy. 
The same place may be the seat of several tides at 
once, which may increase or destroy one another ; 
thus, a small tide is propagated through the Straits 
of Dover as far as the Dutch coast, where it only 
arrives simultaneously with the principal wave, 
which has made the entire tour of Great Britain. 
As regards the progress of theory, Dr Thomas ae of 
Young, whose character as one of the greatest p, Youn 
philosophers of the past age we shall have to eon- 
sider in another chapter, next after Laplace grappled 
with the difficulties of this arduous subject. Em- 
ploying mathematical methods of inferior power but 
greater directness, and taking into account causes of 
local action which Laplace had not ventured to in- 
clude in his analysis, he gradually matured a theory 
adequate to represent many of the results of ex- 
perience, of which Laplace gives no account. 
He distinguishes the results of the foreed and free , (81), 
oscillations of the sea; the former resulting from the gorceq 
direct action of the sun and moon combined with the waves. 
rotation of the earth, and whose periods of rise and 
fall are determined solely by those external causes 
(external, I mean, to the mass of the ocean); the 
Free waves, on the contrary, derived from the former, 
are transmitted with velocities depending on the me- 
chanism of the fluid itself, on its depth, and on the 
resistances arising from friction to which those mo- 
tions are exposed. These all-important modifications 
of the dynamical Theory of the Tides were deduced 
by Dr Young from the general theory of oscillations 
and resistances, and from the laws of fluids detected 
by Dubuat,? and he applied them with no ordinary 
skill to the solution of the problems of tides in 
oceans, estuaries, and rivers. It is an extraordinary 
fact, and not without significance, in the history of 
science, that these researches of Young, published 
anonymously in the Supplement to the Sixth Edition 
of the Encyclopedia Britannica, and in the Seventh 
Edition of this work, and likewise in several jour- 
nals and reviews, so generally escaped notice as to 
have been almost unknown till Dr Peacock, in his 
recently published ‘* Life and Miscellaneous Works 
of Dr Thomas Young” has fortunately recalled 
attention to their existence and their important 
results, 
In doing so, Dr Peacock has communicated with |, ee) 
Mr Airy, whose very valuable article on Tides and qiaes ang 
Waves has been above referred to (78), and has Waves. 
ascertained from him that Dr Young’s researches 
had escaped his notice when he undertook that 
elaborate recension of Laplace’s theory, and made 
those important additions to it to which I have 
1 Encyclopadia Metropolitana, “ Tides and Waves,” art. 117, 5 
2 See Chapter IV., Section 6, where we shall return to some portion of this subject, 
