32 



trinsic or extrinsic, like that ascribed to the moon. But the only 

 power known or ever supposed to act upon the ship in these cir- 

 cumstances, is the attraction of gravitation ; and if that power be 

 sufficient to keep all the ship's particles in the same relative 

 places with respect to the earth's centre, why should it not be 

 sufficient to account for the moon, in such precisely similar cir- 

 cumstances, exhibiting the same phenomenon, that of presenting 

 the same face continually to the centre of her orbit ? In assum- 

 ing the existence of such a rotatory power as that ascribed to the 

 moon, astronomers seem to be not only guilty of the common 

 fallacy of 7ioji causa pro causa; but even to fly directly in the 

 face of Newton's laws of philosophizing, which tell them that 

 they ought never to assume more causes of natural events than 

 are true, and sufficient to account for the phenomena ; and that, 

 of natural effects of the same kind, as in the case of the ship and 

 the moon, the causes are the same. Would not that man be 

 thought mad who should seriously affirm that the earth's cen- 

 tre, along with the radius-vector, revolves about the centre of 

 gravity of every ship that sails round the world, nay of every 

 ship, animal, locomotive engine, and cloud, that describes au 

 arc on, or near to, her surface? Yet, this is just what as- 

 tronomers do say when they affirm that the earth with the 

 moon's radius-vector revolves about the moon's centre during 

 each of her orbital revolutions. Think of the earth's centre per- 

 forming a gyration, or thousands of millions of simultaneous 

 gyrations or oscillations, round about all, and each individually, 

 of the bodies that change place on her surface, loithout ever for- 

 saking its own proper place — the centre ! Yet, say astronomers, 

 the earth herself revolves about the moon, without ever leaving 

 the centre of the orbit ! I think, after this, I may say with con- 

 fidence that the earth and the radius revolve about the moon just 

 as much as the earth's centre revolves about the round-the-world- 

 sailing ship, and no more, that is to say, not at all ; and that the 

 moon, like the ship, revolves in her elliptic pathway, without 

 turning round (m her axis, any more than the ship turns round 

 on its centre of gravity. 



It is to the action of the centripetal force, or of the earth's 

 attraction of gravitation, that I ascribe the fact of the moon's non- 

 rotation. Under the influence of the centrifugal force alone, 

 acting upon its whole mass, while the central axis exclusioeh/ was 

 aff'ected by the centripetal force, the moon would preserve the 

 parallelism already so often mentioned; and astronomers seem 

 all to assume that the moon's circumference, and the particles or 

 molecules of her mass between that and her axis, are entirely free 

 from, and unaff*ected by, the centripetal force of the earth's at- 

 traction ; and it being thus free^ an inherent or intrinsic power 



