TRANSACTIONS OF THE SECTIONS. 
65 
-and springs, and quoted by seamen generally as the half-tide 
mark. Capt. Denham is not as yet prepared to state whether 
some small constant difference might not be found as to the instant 
of the half-elapsed time of spring-tide, high and low water, and that 
of neaps, producing the actual half-range of tide to inches ; but so 
satisfied is he of a closer approximation than is generally allowed, 
that, though he would never propose to adjust soundings to that 
half-tide level , because the mariner would have to make variable 
allowances to ascertain the least water he was to expect in the 
channel before him, yet he would suggest for scientific and frequent 
practical references the desirability of engraving on some rocky 
spot of every harbour, and sheltered portions of coast, the well-de¬ 
fined half-tide level , dated ; for, on the assumption that such a 
level is (no matter what the whole amount of rise and fall differs), 
in the same latitude, equidistant from the earth’s centre, then we 
have a standard of obvious importance to science. By reference to 
this constant level those discrepancies may be adjusted which attend 
engineering operations, designed to cooperate on opposite sides of an 
isthmus, where the vertical range differs, and either high or loio 
water level separately be started from, instead of the mean centre 
of each range , i.e. half-tide level. 
The Rev. Wm. Whewell made the following remarks for the 
purpose of exemplifying the application of physical science to geo¬ 
logical researches. 
1. The permanence of the level of mean water, which Capt. 
Denham has recently proved by trial at Liverpool, suggests the 
proper mode of making such observations on the jDermanence of 
the relative level of land and sea, as were formerly recommended 
by the Association. In tidal seas the level of the ocean must, 
for such a purpose, be estimated with reference, not to the height 
of high or of low water, which is variable on many accounts, but 
to the height of mean ivater. This mean water is to be obtained 
by taking at least two high waters and the intervening low water, 
or two low waters and the intervening high water. A very few tides 
will give a near approximation to the true mean level; but the 
more there are taken, the more accuracy will be obtained. This 
mean level must, of course, for the purposes now spoken of, be re¬ 
ferred to some durable mark in the solid ground. 2. The pheno¬ 
mena of terrestrial magnetism, being apparently connected with the 
internal constitution of the earth, are of interest to the geologist. 
According to the most recent researches of Hansteen the earth has 
four magnetic poles, all of them revolving in the neighbourhood of 
the geographical poles ; and the periods of these revolutions are re¬ 
spectively about 4600, 1740, 1300, and 860 years. These times, 
though long as historical periods, are short compared with many of 
those cycles of which geological researches and astronomical calcu¬ 
lations prove the existence ; and it is impossible not to feel a great 
curiosity respecting the nature of the subterraneous changes which 
take place in such periods. It concerns the geologist therefore, no less 
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