MATTER AND FORCE 157 



that the earth is not perfectly spherical, being flattened at 

 the poles. A body taken towards the equator is at the same 

 time being taken farther from the earth's center. At the 

 equator it is calculated that a body is about thirteen miles 

 farther from the center than when at one of the poles. 



The astronomer explains that gravitation is the great 

 controlling force by means of which the moon is made to 

 move in a circular path about the earth. All bodies when 

 moving tend to move in straight lines by reason of their 

 inertia. It is gravitation that holds the earth itself in a 

 circular path about the sun as a center, preventing it from 

 rushing off into space. The calculated value of this pull of 

 the sun upon the earth in order to hold it to its circular 

 path (orbit) is a value so great as to pass comprehension. 

 When a body is whirled about the hand at the end of a string, 

 it is only necessary to increase the mass of the body and its 

 rate of motion (velocity) to overcome quickly the strength of 

 the string. So great is the mass of the earth, and so enor- 

 mous is its velocity in its orbit (about eighteen miles per second}, 

 that any cable we can conceive fastening earth to sun and 

 replacing gravitation would be as the strands of a spider's 

 web for strength. 



SUMMARY 



When we say that matter is anything that occupies space, it does 

 not follow that we have any considerable knowledge of its real nature. 

 We have merely named one of the readiest ways of identifying matter. 

 The properties of matter are those characteristics by means of which 

 bodies may be recognized and identified. 



Little is known, too, of the real nature of force, but information is 

 available concerning the conditions under which it becomes manifest. 



Force is the cause of all changes occurring in matter. The different 

 ways in which force manifests itself, and the difference in conditions 

 for its manifestation, give rise to a long list of so-called forces, such as 

 heat, light, magnetism, electricity, gravity, cohesion, etc. 



We have been familiar from childhood with the effects of the force 



