54 The Realm of Nature CHAP. 



be no visible mark or sign of the fact. These points, which 

 are called ends or poles, are relatively at rest like the centre 

 of a wheel, and the rate at which a point on the surface of 

 a spinning ball moves is greater in proportion to its distance 

 from them. A body spinning uniformly turns round the axis 

 (NS in Fig. 12) or line joining its poles as a wheel spins 

 round an axle. The two poles of a spinning body are distin- 

 guished from each other by the apparent direction of rotation 

 about them. Looking down on the Earth from above one 

 pole, an observer would see the surface rotating in a direction 

 opposite to that of the hands of a watch, as shown by the 

 thick arrow (Fig. I o), while if he were to look down similarly 

 on the other pole the surface would appear to rotate in 

 the same direction as the hands of a watch do (thick arrow, 

 Fig. 1 1). The end first mentioned is called the North Pole, 

 and the opposite is named the South Pole. The Earth 

 always rotates in one direction, from west to east (arrows 

 in Fig. 12); the apparent difference at the poles is due to our 

 looking from opposite sides. The arrow of Fig. I o appears 

 turning to the left in its flight, that of Fig. i I appears 

 turning to the right, but on holding the page up to the light 

 they are seen to be one and the same. The student should, 

 if possible, make himself familiar with 

 these facts by actual observations on a 

 terrestrial globe. 



89. Ferrel's Law. On a steamer 

 at rest or moving steadily straight for- 

 ward a passenger has no difficulty in 

 walking in a straight line parallel to the 

 planks of the deck, or in any other 

 direction. But if the steamer is turning 

 rapidly to the right, the promenader, 

 ir y m S to keep in a straight line, has 

 deviation of moving the greatest difficulty in preventing him- 

 self from deviating to the left and 

 running against the bulwarks ; or if the 

 steamer is turning to the left he can hardly help de- 

 viating to the right with reference to the planking. The 

 passenger tends to continue walking in a straight line 



