JlAY 30, 1902.] 



SCIENCE. 



847 



of Hipparehus were erroneous— placing 

 the earth at the center, and assuming the 

 motions to be uniform and circular. As to 

 the first of these we must admit that with 

 the evidence then attainable it was the most 

 plausible. In fact as regards the stars we 

 are now, or were until quite recent times, 

 in very much the condition which con- 

 fronted Hipparehus in considering the 

 earth and sun. We know that many of the 

 stars have proper motions, as they are 

 called. In reference to any individual star 

 the appearance would be the same whether 

 we ascribed this motion to the star itself or 

 to our system. The true condition of things 

 is one of the problems which is engaging 

 the attention of the astronomers of to-day. 



As to the attempt to represent the planet- 

 ary motions by combinations of circles, 

 this is precisely what we are constantly 

 doing when we expand the expressions en- 

 tering into our planetary theories in terms 

 of sines and cosines. 



A time came when the primitive system 

 of Hipparehus could no longer be made to 

 harmonize with the results of observation. 

 It was therefore destined to give way to 

 another which may be considered as a sec- 

 ond approximation— that of Copernicus, as 

 perfected by the labors of Kepler. Here 

 the sun is the center of planetary motion; 

 the orbits are ellipses with the sun in one 

 of the foci, but the fundamental cause of 

 these motions, the law of gravity, and the 

 modifications produced by the mutual per- 

 turbations are as yet unrecognized. No 

 place is found for those apparently erratic 

 bodies called comets. 



The next great advance is due to the 

 labors of Newton; by referring all to the 

 law of i;niversal gravitation he was able to 

 explain not only the elliptic motions, but the 

 departures from these curves produced by 

 the mutual perturbations of the planets. At 

 the same time it was shown that the comets 

 which heretofore had been regarded with 



suspicion as erratic visitors, were in fact 

 orderly, law-abiding members of the sys- 

 tem like the planets themselves. 



Is this then a final solution? Is the law 

 of gravity as enunciated by Newton to be 

 regarded as rigorously true, or does it 

 merely form another approximation to the 

 truth? Apparently we may con^der it as 

 absolutely true, though from time to time 

 doubts have arisen on this point. The per- 

 turbations of Jupiter and Saturn, the 

 secular acceleration of the moon's motion, 

 the behavior of Bncke's comet and the 

 motion of Mercury's perihelion have at one 

 time or another given rise to difficulties 

 some of which have never been completely 

 overcome. 



But whether or not the law is rigorously 

 true, no progress whatever has been made 

 toward its physical explanation. In spite 

 of all the ingenuity which has been exer- 

 cised in this direction it remains as much a 

 mystery as in the days of Newton. The 

 true physical explanation is one of the 

 great problems whose solution is still in the 

 future. 



In this development we have noticed a 

 few names which stand out in bold relief. 

 Hipparehus, Copernicus, Kepler, Newton. 

 Are these the only ones to whom credit is 

 due for the creation and development of 

 this department of science? By no means; 

 the astronomer who accumulated observa- 

 tions, the mathematician who helped to per- 

 fect the methods of research, and the stu- 

 dent of mechanics all contributed to this 

 end and are all entitled to a share in the 

 glory of victory. As has been said : If the 

 Greeks had not studied conic sections Kep- 

 ler could not have superseded Hipparehus ; 

 if the Greeks had studied mechanics Kepler 

 might have anticipated Newton. 



Doubtless many branches of science 

 which will occupy the attention of future 

 investigators are still unborn. The status 

 of many of the younger members of this 



