468 HISTORY OF PHYSICAL ASTRONOMY. 



arable distance from our system, the law of attraction according to 

 the inverse square of the distance, prevails. And, according to the 

 practice of astronomers when a law has been established, Tables havc- 

 been calculated for the future motions ; and we have Ephemerides of 

 the revolutions of suns round each other, in a region so remote, that 

 the whole circle of our earth's orbit, if placed there, would be imper- 

 ceptible by our strongest telescopes. The permanent comparison of 

 the observed with the predicted motions, continued for more than one 

 revolution, is the severe and decisive test of the truth of the theory ; 

 aud the result of this test astronomers are now awaiting. 



[2d Ed.] [In calculating the orbits of revolving systems of double 

 stars, there is a peculiar difficulty, arising from the plane of the orbit 

 being in a position unknown, but probably oblique, to the visual ray. 

 Hence it comes to pass that even if the orbit be an ellipse described 

 about the focus by the laws of planetary motion, it will appear other- 

 wise ; and the true orbit will have to be deduced from the apparent 

 one. 



With regard to a difficulty which has been mentioned, that the two 

 stars, if they are governed by gravity, will not revolve the one about 

 the other, but both about their common centre of gravity ; — this cir- 

 cumstance adds little difficulty to the problem. Newton has shown 

 (Princip. lib. i. Prop. 61) in the problem of two bodies, the relation 

 between the relative orbits and the orhit about the common centre of 

 gravity. 



Hoio many of the apparently double stars have orbitual motions ? 

 Sir John Herschel in 1833 gave, in his Astronomy (Art. 606), a list 

 of nine stars, with periods extending from 43 years (77 Coronae) to 

 1200 years (y Leonis), which he presented as the chief results then 

 obtained in this department. In his work on Double Stars, the fruit 

 of his labors in both hemispheres, which the astronomical world are 

 looking for with eager expectation, he will, I believe, have a few more 

 to add to these. 



Is it tvell established that such double stars attract each other accord- 

 ing to the law of the inverse square of the distance ? The answer to 

 this question must be determined by ascertaining whether the above 

 cases are regulated by the laws of elliptical motion. This is a matter 

 which it must require a long course of careful observation to determine 

 in such a number of cases as to prove the universality of the rule. 

 Perhaps the minds of astronomers are still in suspense upon the sub- 

 ject. When Sir John Herschel's work shall appear, it will probably 



