70 INTRODUCTION TO GENERAL SCIENCE 



The proof that the earth revolves on its axis has been rather 

 difficult, and would have been impossible, if it had not been 

 for unlimited travel combined with scientific investigation. 

 Since the poles are nearer the center of the earth, a body would 

 weigh more at the poles than it does at the equator on account 

 of universal gravitation. It has been computed that a body 

 would weigh -5^5- more at the poles than it would at the 

 equator. It has been discovered, by experiment, that a body 

 does weigh T ^ more at the poles than it does at the equator. 

 This is the best proof that the world is in rotation. If you 

 take a stone and tie it to a string and whirl it rapidly, the 

 stone pulls on the string. In the same way a body at the 

 equator, which is moving at the rate of nearly one thousand 

 miles per hour, is thrown out from the earth with a force 

 which resists the weight by -^-J-g-. Near the poles this rotation 

 is very slow, just as the hub of a wheel revolves more slowly 

 than the rim, and a body is not thrown out with the same 

 force, that is, the force is TST less. Thus it is evident that 

 the earth revolves, since y-g-g- -5^-5- = -g-g-g-- 



Revolution around the sun causes seasons. If the axis of 

 the earth were not inclined to the plane in which it revolves 

 around the sun, seasons would be unknown; but the axis of the 

 earth is inclined 23 J to what is called the plane of the earth's 

 orbit. This axis always points in one direction towards 

 the North Star. Thus sometimes the north end of the earth 

 is turned toward the sun ; at other times the south end of the 

 earth's axis is turned toward the sun. Therefore the sun ap- 

 parently moves 23 J north of the equator in the summer time, 

 and 23 J south of the equator in the winter time. 

 Rejerences : 



1. 1002 : 147-152. Proofs of the Rotation of the Earth. 



2. 1002 : 168-172. Proofs of the Revolution of the Earth. 



