ANNIVERSARY ADDRESS OF THE PRESIDENT. xlv 



That the general form of the globe has been almost equally con- 

 stant during all the reach of history, is established, at least for those 

 countries which have a history. 



The boundary of land and sea may be somewhat altered locally, 

 by the growth of new land from the detritus brought by rivers, or 

 by local elevations, whether traceable to earthquakes, or to the 

 general -'reaction of the interior of a planet on its surface*;" but 

 there is no evidence of any general gathering of waters round the 

 poles, or of recession from them, in historic times, as must have 

 happened if the ratio of the equatorial and polar diameters had 

 changed or were changing f. 



Geologists are far removed from the centre of the earth, yet they 

 sometimes indulge in language which implies no small familiarity 

 with the region immediately surrounding it. The interior fluid 

 nucleus of the globe, the waves on the surface of this liquid, the 

 shrinldng of this fiery globe within its warmed but solid crust J, the 

 consequent production of chasms and volcanos of elevation, of alter- 

 nate mountains and vales, the injection of igneous rocks, the meta- 

 morphism of strata, the changes of surface temperature, the displace- 

 ment of land and sea, the destruction of old systems of hfe, and the 

 preparation for newer and more advanced races of plants and animals 

 — these and more than these physical and geological phsenomena, are 

 they not freely and confidently spoken of as necessary corollaries 

 from the admission of the planet having been once fluid through 



* Humboldt uses this expression in Ms great work ' Kosmos.' 

 t The periodicity of deluges — an idea frequently recurring in geological spe- 

 culation — has been made the subject of a treatise by M. LeHon. The well-known 

 facts that the winter and summer halves of the earth's orbit are of unequal length, 

 and that, by the displacement of the apsides of the earth's orbit, this inequality is 

 periodic, and visits the poles alternately, were considered physically by Herr 

 Adhemar (' Die Revolution des Meeres,' Leipzig, 1843). This mathematician 

 conceived that he had found in the inequahty of mean annual temperature occa- 

 sioned by the inequality of the half-yearly periods, a cause for vast accumulation 

 of snows and thickening of glaciers, first round one pole, and afterwards, on the 

 change of the cycle, round the other ; while at some point of time during the 

 change from one condition to the other, there would be a vast and general thaw, 

 occasioning enormous floods, and a readjustment of the earth's centre of gravity 

 to its new centre of figure. The astronomical data may be thus stated : — 



II 



Precessional motion in arc, per annum 50*1 



Motion of apsides in opposite direction 11 '8 



.•. Tropical period (when apsides return to equinox) = 

 360°x60'x60" ^3 OOP + 

 61-9 - ^ 



I This, the usual form of the hypothesis, is controverted by Hennessy, who 

 (Report of Brit. Assoc. 1856) suggests the elevatory pressure exerted by the more 

 oblate fluid nucleus against the solid crust as more favourable to symmetrical 

 fracture than would be a general downward pressure and collapse, according to 

 the views of De Beaumont, the author by whom the systems of fracture in the 

 earth's crust have been most fully examined. The thickness of the earth's crust, 

 inferred by Hopkins to be about one-fourth of the radius, is reduced by Hennessy 

 to a maximum of 600 miles. These authors agree in finding the axis of the 

 earth to be nearly immoveable, and the mean inclination of this axis to the plane 

 of the ecliptic to be also permanent. 



