210 PROGRESS OF SCIENCE. 



of an ETHER within the whole area of our planetary system 

 a view now generally accepted, and, since, even extended to 

 universal space, as Huygens had suggested. Encke showed 

 that ether, offering a certain resistance, and decreasing the 

 comet's velocity, enables the sun to attract the body nearer to 

 itself, to contract therefore its orbit, and cause it to reappear, 

 after passing behind the sun, a few hours sooner (two hours and 

 a quarter) than would be the case otherwise. The decrease 

 has been from 1,21279 in 1786 to 1,210*44 in 1858. The 

 importance of this theory can hardly be exaggerated. It 

 confirms the nebular hypothesis, for the comet must one day 

 be attracted into the sun, and be followed likewise, by virtue 

 of the same law, by all the bodies of the system successively 

 in thousands of ages, of course so that in the process of 

 time our system must be merged into a nebula as it was at 

 starting, and begin from that point a fresh course of evolution- 

 ary cycle. 



1792 1871. Herschel (John) established a catalogue of 



2,OOO DOUBLE AND TREBLE STARS (l8l8); and, going to the 

 Cape of Good Hope, he discovered and mapped 2,500' 

 NEBULA in the Southern Hemisphere thereby completing 

 his father's glorious work (1834 1838). Evidence has thus 

 been furnished, by the observations of the two Herschels on 

 the motions of double stars and the constitution of nebulae, to 

 show that Newton's law of gravitation and Laplace's theory 

 apply to the whole universe ; so that there seem to be 

 features of homogeneity in the structure of all celestial systems 

 which may be looked upon as a demonstration of resistless 

 and unchangeable law a point upon which it is necessary to 

 insist. John Herschel showed that the vast MAGELLANIC 

 CLOUDS a distant group of nebulae and star systems pre- 

 senting marvellous complexity, since they exhibit 28 kinds 

 of nebulae are a portion of the Milky Way of the Southern 

 Hemisphere. But Herbert Spencer has thrown a doubt 

 whether some of these nebulae are as remote from us as great 

 astronomers state, and the reasons he gives are both weighty 

 and cogent for, to put it in a nutshell, if they are resolvable 

 into stars by our telescopes they must be within the limits of 

 our sidereal system, otherwise they would not be resolvable 



