ssct.iv.] DISSERTATION SECOND. 127 



being extended to stars that passed at a more considerable 

 distance from the zenith. 



Even with this addition the observations did not put 

 Bradley in possession of the complete fact, as they only 

 gave the motion of each star in declination, without giving 

 information about what change might be produced in its 

 right ascension. 



Had the whole fact, that is, the motion in right ascen- 

 sion as well as in declination, been given from observation, 

 it could not have been long before the cause was dis- 

 covered. With such information, however, as Dr. Brad- 

 ley had, that discovery is certainly to be regarded as a 

 great effort of sagacity. He has not told us the steps by 

 which he was led to it ; only we see that, by the method 

 of exclusion, he had been careful to narrow the field of 

 hypothesis, a:.d had assured himself that the phenomenon 

 was not produced by any nutation of the earth's axis ; by 

 any change in the direction of the plumb-line, or by re- 

 fraction of any kind. All these causes being rejected, it 

 occurred to him that the appearances might arise from the 

 progressive motion of light combined with the motion of 

 the earth in its orbit. He reasoned somewhat in this man- 

 ner. If the earth were at rest, it is plain that a telescope, 

 to admit a ray of light coming from a star to pass along 

 its axis, must be directed to the star itself. But, if the 

 earth, and, of course, the telescope be in motion, it must be 

 inclined forward, so as to be in the diagonal of a parallelo- 

 gram, the sides of which represent the motion of the earth, 

 and the motion of light, or in the direction of those mo- 

 tions, and in the ratio of their velocities. It is with the 

 telescope just as with the vane at the mast-head of a 

 ship ; when the ship is at anchor, the vane takes exactly 

 the direction of the wind ; when the ship is under weigh, 

 it places itself in the diagonal of a parallelogram, of which 



