314 ILLOGICAL GEOLOGY. 



the Earth in circles, said the earliest observers : led partly 

 by the appearances, and partly by their experiences of cen- 

 tral motions in terrestrial objects, with which, as all circu- 

 lar, they classed the celestial motions from lack of any 

 alternative conception. Without this provisional belief, 

 wrong as it was, there could not have been that compari- 

 son of positions which showed that the motions are not 

 representable by circles ; and which led to the hypothesis 

 of ei3i cycles and eccentrics. Only by the aid of this hy- 

 pothesis, equally untrue, but capable of accounting more 

 nearly for the appearances, and so of inducing more ac- 

 curate observations — only thus did it become possible for 

 Copernicus to show that the heliocentric theory is more 

 feasible than the geocentric theory ; or for Kepler to show 

 that the planets move round the sun in ellipses. Yet 

 again, without the aid of this approximate truth discovered 

 by Kepler, Xewton could not have established that general 

 law from which it follows, that the motion of a heavenly 

 body round its centre of gravity is not necessarily in an 

 ellipse, but may be in any conic section. And lastly, it 

 was only after the law of gravitation had been verified, 

 that it became possible to determine the actual courses of 

 planets, satellites, and comets ; and to prove that, in con- 

 sequence of perturbations, their orbits always deviate, more 

 or less, from regular curves. Thus, there followed one 

 another five provisional theories of the Solar System, 

 before the sixth and absolutely true theory was reached. 

 In which five provisional theories, each for a time held 

 as final, we may trace both the tendency men have to 

 leap from scanty data to wide generalizations, that are 

 either untrue or but partially true ; and the necessity 

 there is for these transitional generalizations as steps to the 

 final one. 



In the progress of geological speculation the same laws 

 of thought are clearly displayed. We have dogmas that 



