64 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Hiss. VI. 



William Herschel." 1 But the most considerable 

 monument to Sir John Herschel' s love of science is 

 the record of his four years' labours for the advance- 

 ment of Sidereal Astronomy at the Cape of Good 

 Hope, where he applied his father's methods of ob- 

 Sir J. Her- servation to the southern hemisphere. His Results 

 schel's ob- O f Astronomical Observations, which fill a large quarto 

 atTh^ Cape volume, and which include " the completion of a 

 of Good Telescopic Survey of the visible Heavens, commenced 

 Hope. j n 1825," form one of the most considerable and 

 most interesting works of our time. The instruments 

 employed were a 20-feet reflector, of 18^- inches aper- 

 ture, and a 7-feet achromatic, with 5 inches of aper- 

 ture. With these the nebulas and double stars of 

 southern skies were examined and measured, and that 

 wonderful " Gauging of the Heavens" completed, of 

 which I have spoken in the account of Sir William 

 Herschel (201). There is an admirable chapter on 

 the apparent magnitude of the stars, to which I shall 

 refer presently, and one on Halley's comet, besides 

 other matters of interest. 



(295.) Since his return to England in 1838, Sir John 

 His high Herschel has withdrawn from the labours of practi- 

 amonffst r ca ^ astronomy, but he continues to advance different 

 his contem- branches of science, and to expound them by his able 

 poraries. and lucid writings in a way which has made his au- 

 thority equally respected by philosophers and by men 

 of the world. The career of Sir John Herschel has 

 been marked by an almost total absence of the ele- 

 ment of ambition, so often a powerful excitement in 

 the pursuit of discovery. Had he sought notoriety 

 and posthumous fame, he would have confined his 

 efforts within a more circumscribed range, But his 

 versatile talents sought their appropriate exercise in 

 all departments of exact science, and even (it is be- 

 lieved) in pursuits widely distinct from these, in 

 natural history, belles lettres, and the fine arts. In 

 all this he no doubt considered simply the useful 

 and pleasurable employment of his mental activi- 

 ties. Truth seemed to him as desirable whether at- 

 tained by the labours of others or by his own ; and 

 in his numerous writings he has expounded these 

 with a zest which a less generous spirit might have 

 reserved for his peculiar achievements. What he 

 may have lost in future fame by this enlargement of 

 his sympathies and interests, he has gained in the re- 

 spect and good-will of all his contemporaries. Sir 

 John Herschel recently filled the post of Master of 

 the Mint, to which, like his illustrious predecessor 

 Newton, he devoted a considerable share of his time. 

 His general eminence as a man of science has been 

 acknowledged by his nomination in 1855 to the dis- 

 tinguished honorary position of one of the eight fo- 

 reign Associates of the French Academy of Sciences. 

 (296.) Orbits of Double Stars. Though not absolutely 

 On the or- fa e fj rst to apply calculation to the orbits of double 

 double stars, this step in their theory may not unfitly be con- 

 stars. 





nected with the name of Sir John Herschel, from the 

 ardour of his researches and the neatness of his 

 methods. To Savary of Paris is due the merit of 

 ascertaining the form and position of the orbit of g 

 Ursse Majoris in 1827, which was followed by a 

 more purely analytical method by M. Encke, and 

 one chiefly graphical by Sir J. Herschel, 2 in which 

 angles of position of the component stars are used 

 nearly to the exclusion of the more doubtful measures 

 of distance. On the whole, these investigations not 

 only confirm Sir William Herschel's anticipations, but 

 render it highly probable that the relative orbits are 

 really ellipses, and consequently that the law of force 

 is that of the inverse square of the distance. The 

 reader will find in Sir J. Herschel's Cape Observa- 

 tions a very curious discussion of the orbit of j Vir- 

 ginis, a remarkable double star, whose interval was 

 in 1783 five seconds and two-thirds, which diminished 

 till 1836, when the two stars appeared united in one, 

 as seen even in the best telescopes. This was the 

 perihelion passage of these two suns, and the angle 

 of position must then have varied (could it have 

 been measured) at the rate of 70 per annum, or 1 

 in 5 days. The following are some of the best as- 

 certained periods of sidereal revolutions in years : 

 Herculis 36M ; g Ursa3 Majoris 61^5 ; aCentauri 

 77 y ; p Ophiuchi 80 or 90?; g Coronse Borealis 600 

 or 700 y . M. Madler, Admiral Smyth, and Mr Hind, 

 have added much to our knowledge of this interest- 

 ing subject. 



Brightness of Stars, and Variable Stars t Sir John (297.) 



Herschel has attempted by an elaborate system of P t , he 

 . , . -A. i i- i T..L brightness 



inter-comparison to assign the correct relative bright- of ^^ 



ness of the stars, and to give precision to the ordi- 

 nary terminology of Magnitudes. His " Method of 

 Sequences" described in his Cape Observations, ap- 

 pears to be one of the happiest specimens of generali- 

 zation which experimental science affords. Whilst 

 regretting the impossibility of here giving even the 

 slightest sketch of it, I cannot but recommend it to 

 the student of natural philosophy as a model of re- 

 search. Having ascertained, in a way independent 

 of every sort of hypothesis, the relative brightness 

 of the stars upon the scale of Magnitudes usually 

 adopted, but which is wholly arbitrary, Sir J. Her- 

 schel proceeds, by properly photometric methods, to 

 give a scientific precision to this notation ; and he 

 arrives at this singular and fortunate conclusion, that 

 by adding a small and constant correction to the re- 

 ceived scale of Magnitudes, the numbers will repre- 

 sent the distances of the respective stars from our 

 system on the supposition of an intrinsic equality in 

 the brightness of the stars themselves. 



This subject naturally includes that of Variable (298.) 

 Stars, which may be divided into those which under- Vanable 

 go periodic or irregular fluctuations, and the latter 

 class may embrace new stars, and stars which have 



1 Quarterly Review, vol. Ixxxv., p. 2. 



2 Astronomical Society'* Memoirs, vol. v. 



