146 Notices respecting New Books. 



Anglo-Gallic, Russian, and Indian arcs, involving in each case the 

 small unknown correction (t) of the latitude of the primary station. 

 Thus the Anglo- Gallic arc gives — 



Semi- major axis (in feet) =20928190+ 1577'7*. 

 Semi- minor axis „ =20847200— 5885-9 t. 



Ellipticity. . = — L_(l +0*0921 t), 



where t is in seconds, and is to be reckoned positive if the deflection 

 be northward. It appears, finally, that if the deflections are taken 

 as l"-37 for the Anglo-Gallic arc, 2"'22 for the Russian, and 0" # 033 

 for the Indian arc, in all cases southernly, values are obtained for the 

 semi- major and minor axes which in no case differ from their mean by 

 so much as 300 ft. Accordingly the Archdeacon's final result is, that 

 the mean figure of the earth is an oblate spheroid, and that its semi- 

 major and minor axes are 20,926,189 feet and 20,855,316 feet re- 

 spectively, and consequently the ellipticity is . Now, inas- 



295*3 



much as the possible existence of deflections of the assumed amount 

 cannot be denied, the investigation shows conclusively that the prin- 

 cipal measured meridian arcs supply data which by no means render 

 it necessary, in the existing state of our knowledge, to suppose that 

 the earth's mean figure is more correctly represented by an ellipsoid 

 than by an oblate spheroid. 



In regard to the second point. What may be compendiously 

 termed the fluid hypothesis is this : — The earth took its form when 

 in a fluid or semifluid state, under the. action of the mutual attrac- 

 tion of its parts combined with their rotation round a common axis. 

 As each point of such a mass must comply with the conditions of 

 fluid equilibrium, the consequence of this hypothesis will be, that, at 

 all events so long as the fluid state exists, the matter composing the 

 earth is arranged in a succession of spheroidal layers having a com- 

 mon centre, the minor axes coinciding with the axis of rotation, 

 their ellipticities decreasing gradually from the surface to the centre ; 

 the density of each layer will be the same throughout, but the den- 

 sities of consecutive strata will gradually increase from the surface 

 towards the centre. It is then assumed that in this state the earth 

 gradually became solid from the surface inwards, a crust (possibly 

 of great thickness) being thus formed over the fluid matter, but 

 without disturbing to any considerable extent the arrangement above 

 explained. 



Independently of considerations drawn from any hypothesis as to 

 the origin of the solar system, or from the facts of geology, it is held 

 that there are good grounds for believing the fluid hypothesis to be 

 true in fact. Thus Laplace says, " La fluidite primitive des planetes 

 est clairement indiqu<§e par l'aplatissement de leur figure, conforme 

 aux lois de l'attraction mutuelle de leurs molecules ; elle est de plus 

 prouvee pour la terre, par la diminution reguliere de la pesanteur, 

 en allant de l'equateur aux poles " (Systhne du Monde, vol. ii. p. 562, 

 cd. 6). Mr. Airy says that " the results of pendulum observations, 



