v The Earth a Planet 67 



of the arrows (Fig. 1 3), trace a circle without allowing the 

 hand to rotate. The diagram shows that the drawing pin, A, 

 if endowed with vision, would see all sides of the pencil (re- 

 presented by the arrows) in succession. Next trace a similar 

 circle, holding the pencil firmly but keeping one side of it, say 

 that covered by the thumb, toward the centre, so that the 



FIG. 14. Revolution of a body rotat- 



FIG. 13. Revolution of a non-rotating j ng once i n t h e same time as it 



body; presenting all sides con- revolves; presenting always the 



secutively to the centre. same s i de to t h e centre. 



drawing-pin can only see the thumb-nail (arrow-head in Fig. 

 14). When the circle is complete the cramped position of 

 the hand will prove that there has been rotation at the wrist. 

 The fact of rotation is shown in the diagram by the arrow 

 pointing successively in every direction. 



103. Differential Attraction and Tides. Since at- 

 traction varies inversely as the square of the distance 

 between the attracting bodies ( 36), it follows that the 

 Moon must exert a greater attractive power on the side of 

 the Earth which is nearest to it than on that which is 8000 

 miles farther away. In consequence of this, the Earth is 

 subjected to a stress tending to lengthen it out toward the 

 Moon. The rigid lithosphere is not perceptibly strained ; 

 the gaseous atmosphere is so readily disturbed by other 

 causes acting irregularly that only the slightest effect from 

 this cause can be detected in it ; but the liquid hydrosphere 

 responds readily and swells into a long low wave, the crests 



