ERA OF THE GREEK AND ALEXANDRIAN SCHOOLS. 



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the sun at the summer, and j at the winter solstice, K his place at the vernal and L at the 

 n K autumnal equinox. This theory provides for the circular orbit and 



uniform velocity of the sun ; for the unequal division of the year, 

 and an apparently unequal rate of motion to a spectator at the 

 earth; for the two arcs formed between K and L are unequal, and 

 it is perfectly obvious that if perceptible to a terrestrial observer the 

 sun would appear to travel fastest at the nearest point of the apsis, H, 

 and slowest at the farthest, G. This scheme is chiefly remarkable for 

 its removal of the earth from the post of honour in relation to the solar 

 orb, which the general voice of antiquity assigned to it with reference to the whole uni- 

 verse, that of being the centre of all the celestial revolutions. 



The idea of his great predecessor was not admitted by Ptolemy, who contended for the 

 immobility and centrical situation of the earth, and attempted to reconcile with these po- 

 sitions, and with the dogmas of perfect circles and uniform velocities, the diverse and 

 complex movements in the heavens. In explaining the irregularities of the planetary 

 courses, he adopted largely the hypothesis of epicycles ; a scheme based upon the general 

 principle, that irregularities of movement may be resolved into a combined series of 

 regular movements a sagacious idea, not however his own, but proposed by some of the 

 earlier Greeks, yet one which he was the first laboriously to apply to the phenomena ob- 

 served. It is demonstrable that two motions, each uniform in itself, may, in combination, 

 appear vexed and tortuous. In illustration, Dr. Nicol says, suppose that a ship were 

 sailing evenly and uniformly along a coast ; it is manifest that a passenger standing on 

 deck would appear to a spectator on shore to move past him also in a regulated and uni- 

 form manner, with the same velocity as the ship ; but if, instead of standing, the passenger 

 should walk to and fro on the vessel's deck, likewise with a constant pace, his actual 

 motion along the coast would clearly seem of the most irregular kind. While walking 

 from stern to prow his motion would appear faster than the ship's; and when walking from 

 prow to stern, it would, on the contrary, appear slower ; nay, if in this latter case the 

 passenger walked faster than the ship sailed, he would appear for a certain time to go 

 along the coast the other way ; so that if the vessel were invisible, and the observer fixed 

 his attention solely on the passenger, he would see very irregular motions ; sometimes 

 accelerated, sometimes retarded, sometimes even retrograde all, however, flowing from 

 u ^ the mere combination of two uniform motions, the sailing of the 



vessel and the walking of the passenger. Now, Ptolemy's theory 

 of epicycles combines two movements in the heavens, that of a 

 centre and a circumference. Thus let E be the earth, and M an 

 c imaginary centre in the heavens: the planet is supposed to be 

 moving around it in the circumference abed all the while the 

 imaginary centre is accomplishing the orbit A B c D. A little 

 attention will soon discover that upon this hypothesis the planet's 

 motion will appear at times direct, stationary, and retrograde to an observer on the earth. 

 But though the assumption satisfies some of the conditions required by the problem of the 

 planetary movements, it goes a very little way towards answering them all, and Ptolemy 

 was obliged to crowd cycle upon cycle, and combine them with eccentrics, in a manner 

 that would be difficult to make intelligible, and useless if attempted. 



It was a blind adhesion to the physics of Aristotle an unquestioning adoption of the 

 false principle respecting circular motion being the law of nature and the earth the 

 centre of the universe that led to this complicated scheme to harmonise its apparent 

 movements. These fundamental errors being admitted, it necessarily followed that the 

 mind must go forth in quest of some plan, to bring into coherence with the maxims 



