Professor Playfulr on Mathematical and Physical Science. 599 

 tliegroundjOr to ascend from it, till tliey 

 attained their own place; but ilit^t, if 

 they were impelled by an oblique force, 

 then their motion became unnalnral or 

 violent, and tended continually to decay) 

 With the heavenly bodies, a^ain, the na- 

 tural motion was circular and uniform, 

 eternal in its course, but perpetually 

 varying in its direction. Thus, by the 

 distinction between natural and violent 

 motion among the bodies of the earth, 

 and the distinction between what we may 

 call the laws of motion in terrestrial and 

 celestial bodies, the ancients threw into 

 all their reasonings upon this fundamen- 



ttfV6lved round the earth. But if a fiery 

 meteor shot across the heavens, — if the 

 flames of a volcano burst forth, — or if an 

 earthquake shook the foundations of the 

 world, terror and curiosity were both 

 awakened ; and, when the former emo- 

 tion had subsided, the latter was sure to 

 become active. Thus, to trace a resem- 

 blance between the events with ivhich 

 the observer was most familiar, and those 

 to which he was less accustomed, and 

 vrhich had excited his wonder, was the 

 first object of inquiry, and produced the 

 first advances towards generalization and 

 philosophy 



This principle, whicli it were easy to "tal subject a confusion and perplexity, 



trace, from tribes the most rude and bar- 

 barous, to nations the most highly re- 

 fined, was what yielded the first attempts 

 toward classification and arrangement, 

 and enabled man, out of individuals, sub- 

 ject to perpetual change, to form certain 

 fixed and permanent objects of know- 

 Jedge, — the species, genera, orders, and 

 classes, into which he has distributed 

 these individuals. By this effort of men- 

 tal abstraction, he has created to hitnself 

 a new and intellectual world, free from 

 those changes and vicissitudes to which 

 all material things are destined. This, 

 too, is a work not peculiar to the philo- 

 sopher, but, in a" certain degree, is per- 

 formed by every man who compares one 

 thing with another, and who employs the 

 terms of ordinary language. 



Another great branch of knowledge is 

 •ccupjed, not about the mere arrange- 

 ment and classification of objects, but 

 about events or changes, the laws which 

 those changes observe, and the causes by 

 which they are produced. In a science, 

 which treated of events and of change, 

 the nature and properties of motion came 

 of course to be studied, and the ancient 

 philosophers naturally enough began their 

 inquiries with tlie definition of motion, or 

 the determination of that in which it con- 

 sists. Aristotle's definition is highly 

 characteristical of the vagueness and ob- 

 ncnrity of his physical speculations. He 

 calls motion '' the act of a being in power, 

 as far as in power," — words to which it is 

 impossible that any distinct idea can 

 ever have been annexed. 



When thelawsof motion wereunknown, 

 the other parts of natural philosophy 

 could make no great advances. Instead 

 of conceiving that there resides in body a 

 natural and universal tendency to perse- 

 vere in the same state, whether of rest, or 

 of motion, they believed that terrestrial 



from which their philosophy never was 

 delivered. 



No information at all could be ob- 

 tained in astronomy, without regular anil 

 assiduous observation, and without in- 

 struments capable of measuring angles, 

 and of measuring time, either directly or 

 indirectly. The steadiness and regula. 

 rity of the celestial motions seemed to 

 invite the most scrupulous attention. On 

 the other hand, as terrestrial objects 

 were always at hand, and spontaneously 

 falling under men's view, it seemed un. 

 necessary to take much trouble to be- 

 come acquainted with them; and, as for 

 applying measures, their irregularity ap- 

 peared to render every idea of such prr)- 

 ceeding nugatory. The Aristotelian phi- 

 losophy particularly favoured this preju- 

 dice, by representing the earth, and all 

 things on its surface, as full of irregula- 

 rity and confusion, while the principles 

 of heat and cold, dryness and moisture, 

 were in a state of perpetual warfare. 

 The unfortunate d. vision of motion into 

 natural and violent, and the distinction, 

 still more unfortunate, between the pro- 

 perties of motion and of body, in the 

 heavens and on the earth, prevented all 

 intercourse between the astronomer and 

 the naturalist, and all transference of the 

 maxims of the one to the speculations of 

 the other, 



DOCTRINE OF CnAVITA-TION. 



Plutarch considers the velocity of the 

 moon's .motion as the cuuse wliich pre- 

 vents that body from falling to the earth, 

 just as the motion of a stone in a sling 

 prevents it from falling to the ground. 

 The comparison is, in a certain degree, 

 just, and clearly implies the notion of 

 centrifugal force ; and gravity may also 

 be considered as pointed at for the cause 

 which gives the moon a tendency to the 

 earth. Here, therefore, a fouodatioa 



bodies tended naturo//^ either to fall to was laid for the true philosophy of tl^e 

 •'" ■>-' ' 4 F a celestia 



