74 PRECESSION. SECT. XI. 



the experiments of Mr. Perkins they appear to be ca- 

 pable of a greater degree of compression than has gen- 

 erally been imagined. 



But a density so extreme is not borne out by astro- 

 nomical observation. It might seem to follow, there- 

 fore, that our planet must have a widely cavernous 

 structure, and that we tread on a crust or shell whose 

 thickness bears a very small proportion to the diameter 

 of its sphere. Possibly, too, this great condensation at 

 the central regions may be counterbalanced by the in- 

 creased elasticity due to a very elevated temperature. 



SECTION XI. 



Precession and Nutation Their Effects on the Apparent Places of the 

 Fixed Stars. 



IT has been shown that the axis of rotation is invari- 

 able on the surface of the earth ; and observation as well 

 as theory prove that were it not for the action of the 

 sun and moon on the matter at the equator, it would 

 remain exactly parallel to itself in every point of its orbit. 



The attraction of an external body not only draws a 

 spheroid toward it, but as the force varies inversely as 

 the square of the distance, it gives it a motion about its 

 center of gravity, unless when the attracting body is sit- 

 uated in the prolongation of one of the axes of the sphe- 

 roid. The plane of the equator is inclined to the plane 

 of the ecliptic at an angle of 23 27' 34"-69 ; and the 

 inclination of the lunar orbit to the same is 5 8' 4 1"' 9. 

 Consequently, from the oblate figure of the earth, the 

 sun and moon acting obliquely and unequally on the dif- 

 ferent parts of the terrestrial spheroid, urge the plane 

 of the equator from its direction and force it to move 

 from east to west, so that the equinoctial points have a 

 slow retrograde motion on the plane of the ecliptic, of 

 50"-41 annually. The direct tendency of this action is 

 to make the planes of the equator and ecliptic coincide, 

 but it is balanced by the tendency of the earth to return 

 to stable rotation about the polar diameter, which is one 

 of its principal axes of rotation. Therefore the inclina- 



