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orange^ if not wholly absorbed by the delicacy of its fragrance, 

 I may think of a ship ; because in an island where they do not 

 grow, a ship is a necessary condition to the presence of an 

 orange ; but would any one ever dream therefore of saying that 

 an orange was a ship, or a ship was an orange ? In both cases 

 there is equally a confounding of things that differ. 



18. He also says that motion is the most obvious of all the affec- 

 tions of matter; but force, as force, is not obvious at all. Again, 

 he says, " Visible motion, or relative change of position in space, 

 is a phenomenon so obvious to simple apprehension, that to 

 attempt to define it would be to render it more obscure.^^ Yet 

 he does define it as " relative change of position ; but what is 

 it that is changed in position ? We cannot predicate change of 

 position of force alone. If there be motion, it must be motion of 

 something ; but force is not a things If we say that motion 

 visibly manifests the exercise of force, all is clear; but when we 

 confound force with motion, we are lost in a chaos of words. 



19. Mr. Spencer attempts to overturn our definition of Force 

 by stating that motion, so far from being distinctly conceivable, 

 as Mr. Grove says, is altogether incomprehensible, and adduces 

 a very peculiar illustration to prove his point : — 



20. A body impelled by the hand is clearly perceived to move, 

 and to move in a definite direction ; there seems at first sight 

 no possibility of doubting that its motion is real, or that it is 

 towards a given point. Yet it is quite easy to show that we not 

 only may be, but usually are, quite wrong in both these 

 judgments. Here, for instance, is a ship which, for simplicity^s 

 sakcj we will suppose to be anchored at the equator with her 

 head to the west. When the captain walked from stem to 

 stern, in what direction does he move ? East is the obvious 

 answer ; an answer which for the moment may pass without 

 criticism. But now the anchor is heaved, and the vessel sails 

 to the west with a velocity equal to that at which the captain 

 walks. In what direction does he now move when he goes 

 from stem to stern ? You cannot say east, for the vessel is 

 carrying him as fast towards the w^est as he walks to the east ; 

 and you cannot say west for the converse reason. In respect 

 to surrounding space he is stationary ; though to all on board 

 the ship he seems to be moving. But, now, are we quite sure 

 of this conclusion ? Is he really stationary ? When we take 

 into account the earth's motion round its axis, we find that 

 instead of being stationary he is travelling at the rate of 1,000 

 miles per hour to the east ; so that neither the perception of one 

 who looks at him, nor the inference of one who allows for the 

 ship's motion, is anything like the truth. Nor, indeed, on 

 further consideration shall we find this revised conclusion to be 



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