38 



KNOWLEDGE. 



[Pebruaky 1, 1897; 



discovery of Neptune. The influence of an exterior planet 

 had been more or less definitely felt through the pertur- 

 bations of Uranus ; but only such mathematical giants as 

 Adams and Leverrier were competent to assign its place 

 at a given epoch. Adams had the priority, but his 

 indications were neglected untU reinforced by those of his 

 French competitor ; and while a leisurely search was 

 slowly progressing at Cambridge, the new planet was 

 picked up, almost at the first glance, by Galle at Berlin, 

 September 23rd, 1846. The possession of a fine star-map 

 gave him, it should be added, a decisive advantage over 

 Challis. 



The earliest information regarding the spectrum of the 

 solar prominences was secured during the Indian eclipse 

 of August 18th, 1868. It proved to consist of a few bright 

 lines of hydrogen and helium : and its discontinuous nature 



EiKpiEOE End uf the 36-inch EyVATuRiAL vi THE Lu'K Observatory, 



enabled Janssen and Lockyer to realize the spectroscopic 

 method of daylight observation at the edge of the sun, by 

 means of which the chromosphere and its prodigious out- 

 lying flames have since been uuintermittently studied. 

 The forms spectroscopically viewed have of late been 

 successfully photographed. The employment for this 

 inirpose, by Hale and Deslandres in 1891, of an apparatus 

 with a double slit, marked the opening of a new chapter 

 in solar physics. A single quality of light is thus effectively 

 isolated ; and both at Chicago and Paris the shadowed 

 " K line " of calcium was chosen as the actinic agent. 

 The same ray is emitted by faculse, which, with its aid, 

 can be photographed all over the sun's surface. 



Since 1837, seven secondary bodies have been added to 

 the planetary system. On October 10th, 1816, Mr. Lassell's 

 famous two-foot speculum showed him Neptune's solitary 

 moon, and, five years later, the two inner Uranian satellites. 

 He espied, moreover, September 19th, 1818, simultaneously 

 •with W. C. Bond on the opposite shore of the Atlantic, 



Hyperion, the seventh in order of distance, and eighth in 

 order of discovery, of Saturn's numerous family. The 

 disclosure to Prof. Hall, August 16th, 1877, of Deimos 

 and Phobos, the dwarf moons of Mars, was the most 

 eminent performance of the Washington twenty-six-inch 

 refractor, which for some years held the telescopic primacy 

 of the world; and it was with a later champion instrument, 

 the Lick thirty-six-inch, that Prof. Barnard descried, Sep- 

 tember 19th, 1892, the small body called, without rhyme 

 or reason, the " fifth satellite of Jupiter." Among the 

 eight principal planets. Mercury and Venus alone remain 

 unattended ; and this circumstance is, more than probably, 

 connected with their peculiar mode of rotation. Each — as 

 Schiaparelli announced in 1889-90 — turns on its axis in 

 the same time that it revolves round the sun, keeping, 

 like our moon, the same face always towards the central 

 body. This agreement was, doubtless, brought about by 

 the grinding down of axial movement through solar tidal 

 friction, the epochs of Instability leading to the birth of 

 satellites being, as a secondary effect, forestalled. The im- 

 portance of tidal friction in cosmogony came to be recog- 

 nized through Prof. Darwin's brilliant research into the 

 origin of the moon. It was presented to the Royal Society 

 in 1879. The predominance of the same influence over 

 the development of stellar systems was ably shown by Dr. 

 See, of Chicago, in 1892. 



Bond's perception of Saturn's "crape ring" led to a 

 revival of curiosity as to the mechanism of the whole set of 

 appendages, and the problem was solved by Clerk Maxwell 

 in 1857. Their formation out of separately-revolving small 

 bodies, demonstrated by him on abstract grounds, was 

 beautifully confirmed by Prof. Keeler's spectrographic 

 experiments in 1895, showing that the radial velocities 

 in the different parts of the system had precisely their 

 theoretical values. 



Ideas regarding the nature of the planets and their 

 central luminary have been profoundly modified during 

 the last sixty years. The four outer planets now rank as 

 "semi-suns," plastic by virtue of their proper heat; 

 while Schiaparelli's discovery of the canals of Mars, and 

 then- mysterious duplications, have tended greatly to 

 impair the analogy, so apparent to Herschel, between that 

 globe and the earth. In 1837, too, Herschel's theory ot 

 the sun reigned without a rival. It now seems as obsolete 

 as that of Anaxagoras. The solar mode of rotation, by 

 surface-drifts slackening towards the poles, discovered 

 by Carrington from spot-movements, and reafiirmed from 

 spectroscopic evidence by Dimer, cannot be that of a solid 

 body ; and all the observed phenomena point to the 

 activity of vertical convection-currents on a prodigious 

 scale, effecting the requisite exchanges of heated and 

 cooled material between the interior and exterior, the 

 expenditure in radiation being supplied by an income from 

 gravitational shrinkage. I'ncertainties of long standing 

 and uncomfortable amount as to the temperature and 

 distance of the sun, have lately been, to a great extent, 

 abolished. The photosphere appears, from experiments 

 made by Messrs. Wilson and Gray in 1894, to be much 

 more than twice as hot as the electric arc, its temperature 

 being about eight thousand degrees centigrade. And 

 Dr. Gill's measures of the minor planet Sappho, in 1889, 

 fix the " astronomical unit,'' or mean radius of the earth's 

 orbit, as ninety-two million eight hundred thousand miles. 

 This value is unlikely to be materially altered. 



The comets of the reign have been in many ways re- 

 markable. Those that appeared in southern latitudes in 

 1848, 1880, 1882, and 1887, followed each other nearly in 

 the same path, and passed close to the sun with tremendous 

 speed, demonstrating by their unretarded course the 



