VOL. LXIII.] PHILOSOPHICAL TRANSACTIONS. 3(^3 



annual parallax of the fixed Stars can ever be discovered, that is, if the diameter 

 of the annual orbit bear a sensible proportion to the distance of the nearest fixed 

 star, it is most likely to be discovered from the observations of Arcturus. The 

 system of the world, considered in an enlarged sense, and agreeable to the idea 

 we may entertain of an all-powerful benevolent Creator, may be taken to occupy 

 the whole abyss of space, and to consist of an assemblage of bodies, having dif- 

 ferent magnitudes, and emitting various degrees and modifications of light. 

 The apparent change of situation visible from the planet which we inhabit, and 

 which revolves round one of the great bodies constituting a part of the general 

 system, as a centre, may be owing either to the motion of our own system in 

 absolute space, or, if our system should be at rest, to a real motion in the stars 

 themselves: whence the angular distances of the stars must vary in proportion 

 to the velocity of those motions, or to the direction of those motions with res- 

 pect to ourselves. I have reason, at present, says Mr. H,, to believe that a 

 small motion may be discovered in the star o ceti, and perhaps in other stars that 

 vary in degrees of brightness, which the diligence of future astronomers will 

 discover, and perhaps in less time than at first sight might seem necessary, when 

 we consider the several improvements which have of late been made in the me- 

 thods of observing the heavenly bodies. 



As the motion of Arcturus in declination, the quantity of which we have thus 

 endeavoured to ascertain, has been often acknowledged, it is matter of wonder 

 that some astronomers, by comparing either the altitude or zenith distance of 

 the sun's limb with Arcturus, without previously settling the quantity of that 

 star's motion in declination, or at least doing it indirectly, should endeavour to 

 determine whether the obliquity of the ecliptic has remained constant, or still 

 continues to diminish, as it should seem to have done for many centuries past, 

 from the observations of successive astronomers. M. Cassini, and M. le Mon- 

 nier, have both practised this method, and are of opinion, that the obliquity of 

 the ecliptic has not altered ; or, if it has altered, that the quantity of its altera- 

 tion is not near so considerable as has been imagined by some celebrated astro- 

 nomers. By observing for several days, before and after the solstice, the altitude 

 or zenith distance of the sun's limb, and that of a star situated near the same 

 parallel, the differences to be remarked in process of time, in the distances of the 

 sun from that star (the motion of the star in declination being allowed for during 

 that interval of time), will be the quantity by which the sun will have approached 

 to or have receded from the star. If the star were absolutely a fixed point, and 

 the observations sufficiently numerous, that, by taking a mean, the necessary and 

 unavoidable errors in observation might either be considerably diminished, or 

 almost annihilated, the method might be practised to great advantage. But as 

 the star Arcturus had a proper motion, and its apparent place was continually 



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