488 PLAN FOm FIXING THE RELATIVE LEVEL OF LAND AND SEA. 
»come so guarded and supported by local 
observations, as to afford a sufficiently 
'Correct guide to be implicitly relied on 
in these speculations. He instanced 
the tides of the Bristol Channel, which, 
in consequence of their excessive mag- 
nitude, afforded magnified representa- 
tions of the phenomena by which the 
deviations become more remarkable. 
At the port of Bristol, the tide rose to 
a height of fifty feet, while towards the 
lower part of the Channel they only 
rose twenty, and along other parts of 
the coast not quite so high. The most 
striking of Mr. Lubbock’s conclusions 
was that by which it appeared that 
the ocean assumed the form of the 
spheroid of equilibrium, according to 
the theory of Bernouilli, but at five 
transits of the Moon preceding the tide 
itself. By the calculations of Mr. Bent, 
however, it would appear, that although 
the observed laws of the tides at 
Bristol might be made to agree with 
Bernouiili’s theory of equilibrium 
tides, by referring them to a certain 
anterior transit, — so far as the changes 
due to parallax were concerned, as also 
as far as those due to declination were 
concerned, — yet it turned out that this 
anterior period itself was not the same 
for parrallax as for declination. The 
two series of changes have not there- 
fore a common origin or a common 
epoch ; so that in fact there is no ante- 
rior period which would give theoretical 
tides agreeing with observed tides ; 
and, therefore, at least the Bristol tides 
do not at present appear to confirm the 
result obtained by Mr. Lubbock from 
the London tides. The learned gentle- 
man then illustrated these views by 
diagrams, by the aid of which he ex- 
plained to the Section the luni-tidal 
intervals, and the curve of semi-men- 
strual in equality — (this latter term, and 
the doctrine connected with it, was 
introduced into the subject of the tides 
by the learned gentleman himself, and, 
as is admitted by all acquainted with 
the subject, with the most valuable 
result). 
Professor Whewell then proceeded 
to the question more immediately before 
him — the proceedings of the Committee 
appointed to fix the relative level of the 
land and sea, with a view to ascertain 
its permanence, or the contrary. He 
observed, that the Committee had not 
taken any active practical steps for the 
important purposes for which they were 
appointed, because they had met with 
many unexpected difficulties requiring 
much consideration. It was, however, 
intended to appoint a Committee for 
the same purposes, who should be fur- 
nished with instructions founded upon 
the views at which the former Commit- 
tee had by their labours and experience 
arrived. One method proposed was, 
that marks should be made along vari- 
ous parts of the coast, which marks 
should be referred to the level of the 
sea ; but here the inquiry met us in the 
very outset— what is the proper and 
precise notion to be attached to the 
phrase the level of the sea ? Was it 
high water-mark or low water-mark 
Was it at the level of the mean tide, 
which recent researches seemed to es- 
tablish ? In hydrographical subjects th a 
level of the sea was taken from low 
water, and this, although in many re- 
spects inconvenient, could not yet be 
dis^iensed with, for many reasons, one 
of which he might glance at — that by 
its adoption, shoals, which were dry at 
low water, were capable of being repre- 
sented upon the maps as well as the 
land. The second method proposed 
appeared to the learned Professor to 
be the one from which the most im- 
portant and conclusive results were to 
be expected. It consisted in accurate- 
ly levelling, by land survey, lines in 
various directions, and by permanently 
fixing, in various places, numerous 
marks of similar levels at the time ; 
by the aid of these marks, at fu- 
ture periods, it could be ascertained 
whether or not the levels, in particular 
places, had or had not changed, and 
thus the question would be settled whe- 
ther or not the land in particular 
localities was rising or falling. Still 
further, by running on those lines, 
which would have some resem- 
blance to the isothermal lines of Hum- 
boldt, as far as the sea coast, and mark- 
ing their extr^emities along the coast, 
a solution would at length be obtained 
to that most important practical ques- 
tion, — what is the proper or permanent 
level of the sea at a given place ? Un- 
til something like this were accomplish- 
ed, the learned Professor e.\ pressed his 
strong conviction of the hopelessness of 
expecting any thing like accuracy in 
many important and even practical cases. 
As an example, he supposed the ques- 
