ON LIGHT. 229 



the tube, however, to be carried along uniformly in any 

 direction by a movement unperceived by himself, the 

 hinder part of it would advance to meet the falling drop ; 

 which would, if the movement in advance were suffi- 

 ciently rapid, cause it to strike against it ; or if not, to 

 emerge at the lower end so far behind the centre as that 

 movement had carried the tube during the time of its 

 passage from end to end. And this deviation would 

 obviously bear the same proportion to the length of the 

 tube that the velocity of the falling drop bore to that of 

 the tube's advance. Judging, then, from this indication 

 alone, if unaware of his own motion j he would conclude 

 the fall of the drop to be inclined backward from the 

 perpendicular by a certain angle but if, suspecting it, 

 he should reverse his movement, and travel with equal 

 speed the contrary way, he would find an equal devia- 

 tion in the contrary direction, and would thus arrive at 

 the certainty that it was to the motion of himself and the 

 tube, and not to any real obliquity in the fall of the 

 drop, that this apparent deviation was owing. And by 

 measuring its angular amount (which would be easy by 

 the help of the marks left by two drops in the opposite 

 circumstances on a screen at the lower end of the tube, 

 and comparing it with the length of the latter), this angle, 

 which might be called the Aberration (from perpendicu- 

 larity) of the apparent line of fall, would inform him of 

 the proportion his own velocity bore to that of the drop 

 in its passage, and, the former being known, would enable 

 him to estimate the latter. 



(12.) All this is a paraphrase of the astronomical phse- 



