^^ M. Arago on Double Stars. 



more matter than our globe. We shall thus penetrate into its 

 internal constitution, although it may be removed from us more 

 than one hundred and twenty billions (120,000,000,000,000) of 

 leagues. 



We now hope, then, however dry this chapter may have ap- 

 peared, that it will be allowed, that we have at least endeavoured 

 to give, and without calculations, some idea of those recondite and 

 fruitful principles, from which astronomers and geometricians 

 deduce results altogether so astonishing* 



-5. The Observations upon the Binary Groups, composed ofindepend- 

 ^ ent Stars, may serve to determine the distance from the earth of one 

 of the Stars of which these Groups are composed. 



We now proceed to endeavour to furnish, in this chapter, an 

 elementary view of the methods which astronomers have em- 

 ployed to ascertain the distances of the stars from the earth. 

 We can thus bring under review the advantage of that one of 

 those methods which is grounded upon the observation of the 

 double stars. 



Let us suppose AB an indefinite horizontal straight line, 

 upon all the points of which, a person making observations may 

 transport himself, furnished with an instrument which astrono- 

 mers employ in measuring the angles contained in vertical planes, 



• Mathematically speaking, the velocity with which a bullet falls towards 

 the earth, depends on the mm of the masses, both of the earth and bullet. 

 The fall of the earth towards the sun is also determined by the mm of the 

 masses of the sun and the earth ; it is from the result of these sums of 

 masses, and not merely the effect of an isolated mass, that the calculation is 

 effected. But it is evident, that, on account of the extreme smallness of the 

 bullet compared with the earth, and of the earth compared with the sun, that 

 we may, without any appreciable error, adopt the hypothesis insisted upon in 

 the text. It is not, however, the same with regard to the double stars. 

 Sometimes the satellite star differs'but very little from the central one (at least 

 if we may judge from the intensity of the lustre), so that we could not regard 

 the result of the calculation given above as giving the sum of the masses of 

 the two stars. 



