ASTRONOMY. 167 



tion that can reasonably be ascribed to the lat- 

 ter will explain the irregularities observed* 



In the present case, there is not a certainty that the 

 spectator, (or the Earth), is at rest : the want of 

 any information of his motion from terrestrial ob- 

 jects affords none, as, according to a principle al- 

 ready explained, the motions of bodies among 

 one another, are nowise affected by any motion 

 which is common to them all. 



166. When a spectator moves, without being 

 sensible of it, he necessarily transfers his own 

 motion to the objects around him, estimating it 

 in a direction opposite to that in which he has 

 actually moved. 



Suppose an object to move from A to B, (fig. 18.), 

 while an observer, unconscious of his own motion, 

 is carried from C to D. From B draw BF equal 

 and parallel to CD, but extending from B the op- 

 posite way that CD does from C, and join AF ; 

 AF will be the apparent path of the body, and F 

 its apparent place, at the time when the spectator 

 is really in D, and the body in B. 



If the velocity of the observer had been the same, 

 but in an opposite direction, CD', BF' being 

 drawn == CD', but in the opposite direction, the 

 apparent motion of A would have been in the line 

 AF' 



This 



