128 DISSERTATION SECOND. [tAKtn. 



one side represents the velocity of the ship, and the other 

 the velocity of the wind. If, instead of the vane, we con- 

 ceive a hollow tube, moveable in the same manner, the 

 case will become more exactly parallel to that of the teles- 

 cope. The tube will take such a position that the wind 

 may blow through it without striking against the sides, 

 and its axis will then be the diagonal of the parallelogram 

 just referred to. 



The telescope, therefore, through which a star is viewed, 

 and by the axis of which its position star is determined, 

 must make an angle with the straight line drawn to the 

 star, except when the earth moves directly upon the star, 

 or directly from it. Hence it follows, that if the star be 

 in the pole of the ecliptic, the telescope must be pointed 

 forward, in the direction of the earth's motion, always by 

 the same angle, so that the star would be seen out of its 

 true place by that angle, and would appear to describe a 

 circle round the pole of the ecliptic, the radius of which, 

 subtended at the earth, an angle, of which the sine is to 

 unity, as the velocity of the earth to the velocity of light. 

 If the star be any where between the plane of the eclip- 

 tic and the pole, its apparent path will be an ellipse, the 

 longer axis of which is the same with the diameter of the 

 former circle, and the shorter equal to the same quantity, 

 multiplied by the sine of the star's latitude. If the star 

 be in the plane of the ecliptic, this shorter axis vanishes, 

 and the apparent path of the star is a straight line, equal 

 to the axis just mentioned. 



Bradley saw that Romer's observation concerning the 

 lime that light takes to go from the sun to the earth, gave 

 a ready expression for the velocity of light compared with 

 that of the earth. The proportion, however, which he 

 assumed as best suited to his observations was somewhat 

 different ; it was that of 10313 to 1, which made the ra- 

 dius of the circle of aberration 20', and the transverse 



