Jax. 0, 1885.] 



♦ KNOWLEDGE ♦ 



25 



diminution in the rate of advance until the planet again 

 seems momentarily at rest ; and, finally, a graduully in- 

 creasing retrogradation up to that observed at fir.-t. And, 

 obviously, the duration of the advancing motion is consider- 

 ably greater than that of the retrogradation, precir^ely as 

 the arc t E <•' is greater than the arc < ' E' e. Clearly, 

 it only requires that the path of the planet should not be 

 in the same pUuie as the earth's for this alternation of 

 advance and recession to produce an apparently looped path ; 

 ■while, according to the position of the planet with respect 

 to the points in which its path intersi cts the piano of the 

 eiirth's path, the shape of the loops will vary through all 

 the forms described earlier in the chapter. 



Again, it is clear that, so far as planets external to the 

 earth are concerned, the relation of the loops in size and 

 number can be at once accounted for. Planets very far 

 from the earth would obviously exhibit a relatively small 

 loop, and many loops in their complete circuit of the 

 heavens. On the other hand, planets not so far from the 

 earth would exhibit larger loops and fewer in the circuit, 

 or even but one loop in more than one circuit. 



As for planets within the orbit of the earth, a very 

 simple device will at once exhibit their relations without 

 any new discussion. To an interior planet, the earth if, 

 of course, an exterior planet, and therefore she describes in 

 appearance just such loops as exterior planets have been 

 shown to describe. But, if one conceives an indefinitely 

 long line passing always through the centres of the earth 

 and an interior planet, it is obvious that, whatever path one 

 end of the line describes on the heavens as earth and 

 planet travel on their orbits, a similar path will the other 

 end describe on the opposite region of the heavens. In 

 other words, an interior planet describes an apparent path 

 precisely simUir to that which the earth, as seen from that 

 planet, would appear to describe, or, as already shown, a 

 series of loops resembling those described by an exterior 

 planet. 



Copernicus was not able to account for all the pecu- 

 liarities of planetary motion. In fact, when once he con- 

 sidered minor peculiarities, he fouiid precisely the same 

 difficulties which had perplexed the Ptolemaists. He had 

 got rid of the grand difficulty of the Ptolemaic theory — 

 the conception of a set of moviog points around which, 

 instead of around real centres, the planets were required to 

 move. But all the minor difficulties yet remained. It 

 was Kepler's task to remove them. 



The task was a hopeful one, however, since there was no 

 longer an ill-conditioned h3'pothesi3 as the ba.sis of the 

 whole sys'em of astronomy. It is shown in every book of 

 astronomy how Kepler mastered all the minor difficulties 

 by det':CtiDg the simple laws that (1), the planets move in 

 ellipses round the sun, which lies in a focus of each path, 

 and (2), the line drawn from the sun to a planet passes 

 over equal areas in equal times. 



I must not, however, altogether neglect the system sug- 

 gested V>y Tycho Brahe, according to which the earth was 

 replaced at the cen're of the universe, the sun circling 

 round v, but, being himself the centre of all the planetary 

 motions, Tycho was unfortunate in coming after Copernicus. 

 Had he lived a few centuries earlier, his theory would 

 most probably have found acceptance, and, had it once 

 been established, Copernicus would have found it very dif- 

 ficult to establish his own views. I have no hesitation in 

 pronouncing the Tychonic system a masterpiece of inge- 

 nuity. It accounts quite as well as the Copernican for all 

 the celestial motions known in Tycho's day. It required 

 no modification other than one reeembling that by which 

 Kepler improved the Copernican theory, to account for 

 observed motions as exactly as on the Keplerian theory. 



For the oty> cts observed, if tlid sun travelled in an ellipse 

 roiuid the earth, and the planets, in their proper ellipses, 

 round the sun, would be exactly the same as those ob- 

 served if the sun were the centre of all the motions, the 

 earth travelling in itn i)ropcr ellipse about him. And there 

 was this furtlier pi'int in favour of the Tychonic system — 

 that it got rid of the great difficulty (pointed out by Brahi 

 himself) that if the earth circled in an enormous orbit 

 round the sun, the stars ought to be seen to shift in position 

 unless their distances were inconceivably great. 



So that, as a matter of fact, the Copernican theory had 

 nothing in its favour but the great suiieriority in the sun's 

 volume -nhich was suspected even in Tycho Brahe's day. 

 So far as this superioiity rendered it likely that the sun 

 and not the earth, was the centre of the universe, the 

 theory of Copernicus seemed the better. So far as the 

 point considered in the last sentence of the preceding para 

 graph was concerned, the theory of Tycho Brahd seemed 

 the better. Kepler, by changing the circles of Copernicus 

 into ellipses, did not alter the relative position of the two 

 theories, since his ellipses were quite as available for the 

 Tychonic system as for the Copernican. 



It was only when Kewton established the theory of gravi 

 tation that the Copernico- Keplerian theories which had 

 been already adopted could be said to have been demon- 

 strated. Into the proof of universal gravitation it would 

 be out of place to enter here. Suffice it to say that the 

 proof was made com])lete even by Newton himself, though 

 since his days a thousand new evidences have been revealed 

 which have placed the theory absolutely beyond quebtion, 

 save by those too ignorant to understand the matter. It 

 will be seen at once how it affects the question which had 

 been at issue between Tycho and the Copernicans. Obviously 

 if the sun is the centre of the planetary motions, he, and 

 not the earth, is the overruling attractive body ; the 

 earth, then, must travel around him, and not he around 

 the earth. 



But although Newton, in establishing the law of gravity, 

 had established the Copernican system, the most striking 

 proof of that system was that obtained when Bradley 

 discovered and explained the aberration of light. 

 (To he continued.) 



DIXOSAITRS.* 



THE first naturalists who described reptiles as crawling 

 animals would certainly have modified the opinion 

 that they expressed had they known the strange creatures 

 whose history we are about to sketch. 



These animals, which are designated as ornithosceliaiis 

 or dinosaurians, partake, by certain characteristics of their 

 organisation, of the nature of mammals, bird?, and reptiles 

 properly so called, while at the same time exhibiting cha- 

 racters that are proper to themselves. They seem to bridge 

 over the gap which in present nature separates the most 

 perfect of the reptiles, the crocodiles and the tortoises, from 

 the lower mammals— the marsupials— and from such birds 

 as the ostrich, emu, and cassowary. They are so far re- 

 moved from the reptiles that we have to form a distmct 

 subclass for them equal in value to that which is admitted 

 for reptiles of the present time. 



The differences that they present from our reptiles are 

 much greater than those that we find between tortoises and 

 serpents, for example, to merely cite the two extreme terms 

 of the series. 'We know nothing of the dino.5aurs except 

 their skeleton. It is probable that if i t were permitted us 



* Science et Nature. 



