Section of Geology and Geography, 489 



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 low 

 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 permanence 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 water. 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, 1306, 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 

 than the physical philosopher, to further the progress of our know- 

 ledge of terrestrial magnetism. 3. The heat of the interior parts of 

 the earth has always been treated of by those who have established 

 the theory of heat upon mathematical principles. They have 

 hitherto considered it as proved, upon such principles, that the in- 

 crease of temperature of the substance of the earth as we descend, 

 proves the reality of an original heat. But M. Poisson, in his 

 Third Series. Vol. 7. No. 42. Dec. 1835. 3 R 



