On the Internal Fhddity of the Earth, 4>U7 



D^TOTiCES OIF 3^^:E3:M:oII^S- 



I. — On the HyroTHEsis of the Internal Fluidity op the 

 Terrestrial Globe. 



By M. Delaunay, Academie des Sciences, Stance du 13 Juillet 1868. 



[Communicated by David Forbes, F.R.S., &c.] 



APEOFOUND study of the several circumstances of the form, 

 composition, and temperature of the materials which constitute 

 the surface of the terrestrial globe leads to the admission that its 

 interior possesses a high temperature, and, consequently, that the 

 dijBferent substances of which it is composed are in great part in a 

 state of fusion, so that the globe itself is essentially a liquid mass 

 covered by a solid crust of but little thickness when compared with 

 its diameter. 



A formidable objection to this opinion was brought forward nearly 

 thirty years ago by Mr. Hopkins in a series of memoirs inserted in 

 the Philosophical Transactions of the Koyal Society of London for 

 the years 1839, 1840, and 1842. This objection, which is based 

 upon the consideration of two astronomical phenomena, ^kiQ preces- 

 sion and nutation, is as follows : — It is well known that the preces- 

 sion and nutation taken conjointly consist in a change of direction 

 experienced by the axis of rotation of the earth. "Without the pre- 

 cession and nutation, the axis of the earth would always remain 

 parallel to itself, and if prolonged would always pierce the celestial 

 dome in exactly the same point, at least if the dimensions of the 

 earth's orbit be disregarded when compared with the distance which 

 separates it from the stars. 



In consequence, however, of the precession and nutation, the 

 earth's axis becomes more and more inclined from the direction 

 it previousl}'' possessed ; the point in which it pierces the celestial 

 dome, to which the name of the Pole is given, displaces itself slowly 

 and by degrees amongst the stars, the precession causing it to 

 describe a circle parallel to the ecliptic, whilst the nutation causes 

 it to move in a very small ellipse, having for centre the position 

 whicli it would have occupied if influenced by the precession alone. 



This continual change in the direction of the earth's axis of rota- 

 tion has been connected in a most happy manner with the grand 

 law of universal gravitation, — Newton having demonstrated that the 

 movement of precession follows as a consequence of the flattening 

 of the earth. 



The attraction which the sun exercises on the entire mass of the 

 terrestrial globe would have no influence whatever on the rotary 

 motion of the globe round its centre if the globe itself were spherical 

 and homogeneous, or if it were made up of concentric and homo- 

 geneous spherical layers. 



In consequence, however, of the swelling out of the globe along 

 the equator this is not the case : the action of the sun upon the sort 

 of pad formed by this equatorial swelling, causes little by little, a 



