iict. ii.] DISSERTATION SECOND. 47 



the resolution of forces is necessary, that principle being 

 then entirely unknown. Hence the force necessary to sus- 

 tain a body on an inclined plane, is incorrectly determined 

 by Pappus, and serves to mark a point to which the me- 

 chanical theories of antiquity did not extend. 



In another department of physical knowledge, Astrono- 

 my, the endeavours of the ancients were also accompanied 

 with success. I do not here speak of their astronomical 

 theories, which were, indeed, very defective, but of their 

 discovery of the apparent motions of the heavenly bodies, 

 from the observations begun by Hipparchus, and continued 

 by Ptolemy. In this their success was great ; and while 

 the earth was supposed to be at rest, and while the instru- 

 ments of observation had but a very limited degree of ac- 

 curacy, a nearer approach to the truth was probably not 

 within the power of human ingenuity. Mathematical rea- 

 soning was very skilfully applied, and no men whatever, in 

 the same circumstances, are likely to have performed more 

 than the ancient astronomers. They succeeded, because 

 they were observers, and examined carefully the motions 

 which they treated of. The philosophers, again, who stu- 

 died the motion of terrestrial bodies, either did not observe 

 at all, or observed so slightly, that they could obtain no ac- 

 curate knowledge, and, in general, they knew just enough of 

 the facts to be misled by them. 



The opposite ways which the ancients thus took to study 

 the Heavens and the Earth, observing the one, and dream- 

 ing, as one may say, over the other, though a striking incon- 

 sistency, is not difficult to be explained. 



No information at all could be obtained in astronomy, 

 without regular and assiduous observation, and without in- 

 struments capable of measuring angles, and of measuring 

 time, either directly or indirectly. The steadiness and re- 

 gularity of the celestial motions seemed to invite the most 



