ULTIMATE SCIENTIFIC IDEAS. 57 



that they leave us nothing but a choice between opposite 

 absurdities. 



§ 17. 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 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 sake, we will suppose to he anchored at the equa- 

 tor with her head to the West. When the captain walks 

 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 West 

 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 1000 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 

 much better. Eor we have forgotten to allow for the 

 Earth's motion in its orbit. This being some 68,000 miles 

 per hour, it follows that, assuming the time to be midday, 

 he is moving, not at the rate of 1000 miles per hour to the 

 East, but at the rate of 67,000 miles per hour to the West. 

 !N"ay, not even now have we discovered the true rate and the 

 true direction of his movement. With the Earth's progress 

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