278 DISCOURSE ON THE STUDY 



particular system of sun and planets, or of the earth 

 itself, as the unit of a scale on which it might be 

 measured. But although many have imagined that 

 their observations afforded grounds for the decision 

 of this interesting point, it has uniformly happened 

 either that the phenomena on which they relied 

 have proved to be referable to other causes not 

 previously known, and which the superior accuracy 

 of their researches has for the first time brought to 

 light ; or to errors arising from instrumental im- 

 perfections and unavoidable defects of the observ- 

 ations themselves. 



(309.) The only indication we can expect to ob- 

 tain of the actual distance of a star, would consist 

 in an annual change in its apparent place corre- 

 sponding to the motion of the earth round the sun, 

 called its annual parallax, and which is nothing 

 more than the measure of the apparent size of the 

 earth's orbit as seen from the star. Many observers 

 have thought they have detected a measurable 

 amount of this parallax ; but as astronomical instru- 

 ments have advanced in perfection, the quantity 

 which they have successively assigned to it has 

 been continually reduced within narrower and nar- 

 rower limits, and has invariably been commensurate 

 with the errors to which the instruments used 

 might fairly be considered liable. The conclusion 

 this strongly presses on us is, that it is really a 

 quantity too small to admit of distinct measurement 

 in the present state of our means for that purpose ; 

 and that, therefore, the distance of the stais must 

 be a magnitude of such an order as the imagination 

 almost shrinks from contemplating. But this in- 



