The Realm of Nature 



CHAP. 



point B in 60 latitude. But the line CE is nearly 4000 

 miles long, while the line AB is not 2000 miles ; therefore 

 during the time of one rotation the point E is carried through 

 more than 24,000 miles, while the point B is carried through 

 little over 12,000 miles, and the points N and S are at rest. 



The rate of movement of the 

 Earth's surface by rotation is 

 called its tangential velocity, 

 and diminishes from over 1000 

 miles an hour at the equator 

 to 500 miles an hour at 60, 

 and o at the poles. A body 

 resting on the Earth's surface 

 has a tendency to fly away at 

 a tangent, like a stone in a 

 sling, and the force of gravity 

 is partly employed in prevent- 

 ing this. The centrifugal force 



FIG. 12. Diagrammatic Section of ( 51) makes bodies weigh 



CN,S^cftSS; ; N^' ; less at the equator than at the 

 AB, perpendicular to axis; o, a poles, reinforcing the change 



point in 45 N. lat. ; B, B, points J 



m 60 lat. due to the fact that the equator 



is more distant than the 



poles from the Earth's centre (38). If the Earth rotated 

 seventeen times more rapidly than it does the centrifugal 

 force would be equal to gravity, and if it rotated in the least 

 faster the equatorial part of the Earth would split off like 

 the edge of a burst grindstone. The increase of tangential 

 velocity with length of radius enabled the fact of the Earth's 

 rotation to be proved in the seventeenth century by dropping 

 a weight from the Leaning Tower of Pisa, and observing 

 the distance of its fall to the east of the perpendicular line. 

 The weight was moving eastward on the top of the tower 

 more rapidly than the base of the tower, and retained its 

 original motion in consequence of inertia. 



94. Measurement of Rotation. The period which 

 elapses between the Sun crossing the meridian or north 

 and south line of a place on two successive occasions is 

 called a day, and is divided into 24 equal parts or hours ; 



