OF NATURAL PHILOSOPHY. 267 



(295.) An elegant writer*, whom we have before 

 had occasion to quote, has briefly and neatly ac- 

 counted for the confused notions which so long pre- 

 vailed respecting the constitution of our system, 

 and the difficulty experienced in acquiring a true 

 notion of the disposition of its parts. " We see it," 

 he observes, "not in plan, but in section." The 

 reason of this is, that our point of observation 

 lies in its general plane, but the notion we aim at 

 forming of it is not that of its section, but of its plan. 

 This is as if we should attempt to read a book, or 

 make out the countries on a map, with the eye on a 

 level with the paper. We can only judge directly 

 of the distances of objects by their sizes, or rather 

 of their change of distance by their change of size ; 

 neither have we any means of ascertaining, otherwise 

 than indirectly, even their positions, one among the 

 other, from their apparent places as seen by us. Now, 

 the variations in apparent size of the sun and moon 

 are too small to admit of exact measure without the 

 use of the telescope, and the bodies of the planets 

 cannot even be distinguished as having any distinct 

 size with the naked eye. 



(296.) The Copernican system once admitted,how- 

 ever, this difficulty of conception, at least, is effectu- 

 ally got over, and it becomes a mere problem of geo- 

 metry and calculation to determine, from the observed 

 places of a planet, its real orbit about the sun, and 

 the other circumstances of its motion. This Kepler 

 accomplished for the orbit of Mars, which he ascer- 

 tained to be an ellipse having the sun in one of its 

 foci ; and the same law, being extended by inductive 

 * Jackson, Letters on Various Subjects, &c. 



