TRANSLATORY MOTION. 137 



solar system. The existence of these laws is revealed to us 

 by many phenomena, as for instance by the motion of double 

 stars, and Jby the amount of retarded or accelerated motion in 

 different parts of their elliptic orbits. Human inquiry need 

 no longer pursue this subject in the domain of vague con- 

 jecture, or amid the undefined analogies of the ideal world ; 

 for even here the progress made in the method of astronomical 

 observations and calculations has enabled astronomy to take 

 up its position on a firm basis. It is not only the discovery 

 of the astounding numbers of double and multiple stars re- 

 volving round a centre of gravity lying without their system, 

 (2800 such systems having been discovered up to 1837) but 

 rather the extension of our knowledge regarding the funda- 

 mental forces of the whole material world, and the proofs we 

 have obtained of the universal empire of the laws of attraction, 

 that must be ranked among the most brilliant discoveries of 

 the age. The periods of revolution of coloured stars present 

 the greatest differences ; thus, in some instances, the period 

 extends to 43 years, as in 77 of Corona, and in others to several 

 thousands, as in 66 of Cetus, 38 of Gemini, and 100 of Pisces. 

 Since Herschel's measurements in 1782, the satellite of the 

 nearest star in the triple system of of Cancer has completed 

 more than one entire revolution. By a skilful combination of 

 the altered distances and angles of position,* the elements of 

 these orbits may be found, conclusions drawn regarding the 

 absolute distance of the double stars from the Earth, and 

 comparisons made between their mass and that of the Sun. 

 Whether, however, here and in our solar system, quantity of 

 matter is the only standard of the amount of attractive force, 

 or whether specific forces of attraction proportionate to the 

 mass, may not at the same time come into operation, as Bessel 

 was the first to conjecture, are questions whose practical solu- 

 tion must be left to future ages.f When we compare our 



* Savary, in the Connaissance des Terns, 1830, pp. 56 and 163. Encke, 

 Berl. Jahrb., 1832, s. 253, &c. Arago, in the Annuaire, 1834, pp. 

 260, 295. John Herschel, in the Memoirs of the Astronom. Soc., vol. v. 

 p. 171. 



f Bessel, Untersuchung des Theils der planetarischen Stb'rungen, 

 welche mis der Bewegung der Sonne entstehen, (An Investigation of the 

 portion of the Planetary Disturbances depending on the motion of the 

 Sun) in^l^. der Berl. AJcad. der Wissensch, 1824 (Mathem. Classe\ 



