DISCOVERIES IN THE CELESTIAL SPACES. 691 



fchat of the fall of bodies, or the tendency of all substances, 

 whether heavy or light, to reach the ground.* The idea 

 conceived by Copernicus, and more clearly expressed by 

 Kepler, in his admirable work De Stella Martis, who even 

 applied it to the ebb and flow of the ocean, received in 1666 

 and 1674 a new impulse and a more extended application 

 through the sagacity of the ingenious Robert Hooke ;f New- 

 ton' s theory of gravitation, which followed these earlier ad- 

 vances, presented the grand means of converting the whole of 

 physical astronomy into a true mechanism of the heavens. \ 



Copernicus, as we find not only from his dedication to the 

 Pope, but also from several passages in the work itself, had a 

 tolerable knowledge of the ideas entertained by the ancients 

 of the structure of the universe. He, however, only names 

 in the period anterior to Hipparchus, Hicetas (or, as he 

 always calls him Nicetas,) of Syracuse, Philolaus the Pytha- 

 gorean, the Tima3us of Plato, Ecphantus, Heraclides of Pon- 

 tus, and the great geometrician Apollonius of Perga. Of 

 the two mathematicians, Aristarchus of Samos, and Seleucus 

 of Babylon, whose systems came most nearly to his own, 

 he mentions only the first, making no reference to the 

 second. It has often been asserted that he was not ac- 



* Job. Philoponus de creatione mundi, lib. i. cap. 12. 



t He subsequently relinquished the correct opinion (Bre water, Jtfar- 

 tyrs of Sciences, 1346, p. 211); but the opinion that there dwells in the 

 central body of the planetary system the sun a power which governs 

 the movements of the planets, and that this solar force decreases either 

 as the squares of the distances or in direct ratio, was expressed by 

 Kepler, in the Harmonices Mundi, completed in 1618. 



J See Cosmos, pp. 28 and 45. 



See op. tit. p. 544. The scattered passages to be found in the work of 

 Copernicus, relating to the Ante-Hipparchian system of the structure 

 of the universe are, exclusive of the dedication, the following : lib. i. 

 cap. 5 and 10 ; lib. v. cap. 1 and 3 (ed. princ. 1543, p. 3,b; 7,b ; 8, b; 

 133, b; 141 and 141, b; 179 and 181, b). Everywhere Copernicus 

 shows a predilection for, and a very accurate acquaintance with, the 

 views of the Pythagoreans, or, to speak less definitely, with those which 

 were attributed to the most ancient among them. Thus, for instance, 

 he was acquainted, as may be seen by the beginning of the dedica- 

 tion, with the letter of Lysis to Hipparchus, which, indeed, shows that 

 the Italian school in its love of mystery, intended only to communicate 

 its opinions to friends, " as had also at first been the purpose of Coper- 

 nicus." The age in which Lysis lived is somewhat uncertain ; he i* 

 2 Y 2 



