66 ELEMENTS OF SCIENCE 



A line drawn from it to the centre of the earth, must 

 always move in the same plane, and in such a way as to 

 pass over equal areas in equal times. Such a line is 

 called a " radius vector" 



But the force of gravity between bodies does not alone 

 draw everything at or near the earth's surface, towards 

 the earth's centre, it also draws every existing body 

 towards every other, although its action between small 

 bodies is too feeble to be easily observed. 



Each body thus draws towards itself every other body 

 with a force of gravity which varies directly as its mass 

 and inversely as the square of its distance from the body 

 it attracts. 



On account of the inertia of matter, or its absolute 

 indifference to motion, every separate body on the earth's 

 surface would, by the force of the earth's rotation, be 

 projected a.nd continue on ward i in a straight line from 

 its surface in a tangent* were it not for the force of 

 gravity which keeps it in its place. The passive tendency 

 to continue onwards in a tangential straight line or 

 " to fly oft'" is, as before said, termed "centrifugal 

 force," while the action of gravity which conflicts there- 

 with is called "centripetal force." These two forces 

 arise together, and illustrate that bifold nature of forces 

 implied in Newton's third law.f 



Now, as we all know, the earth revolves on its axis 

 once in every twenty-four hours. 



The weight of any object, then, is that portion of its 

 gravity over and above that which is required to prevent 

 its "flying off" (owing to our globe's rotation), and to 

 retain it on the earth's surface. Had bodies no more 



* A tangent is a line touching the circumference of a circle 

 and at right angles to the diameter of the circle at tLe point 

 of contact. t See ante, p. 56. 



