8 GRAVITY, AND ITS EFFECTS. 



Again, the direction of rest taken by a string 

 to which a weight is hung (the Plumb-line), shows 

 the position of the horizontal plane ; for this direc- 

 tion can be no other than that of gravity : because, 

 so long as the string does not lie in the line of 

 this force, the weight attached to it, which is so 

 very easily moved, cannot come to rest. 



Heavy bodies, free to move, fall in the direction 

 of the plumb-line ; that is, their line of motion is 

 perpendicular to the horizontal surface of the 

 earth. They fall the more quickly, the greater the 

 power which draws them down. They fall least 

 quickly at the equator, more so as the latitude 

 increases, and most quickly at the poles. 



The increase, from the equator to the poles, of 

 the speed of fall cannot be learned by the imme- 

 diate observation of bodies freely falling, since this 

 motion is everywhere too swift to allow of truly 

 measuring the spaces fallen through in given 

 times. However, we can make our observations 

 with the greatest certainty and accuracy, by means 

 of the Pendulum, or swinging plumb-line ; for it is 

 nothing but the force of gravity that makes the 

 pendulum fulfil its swings, or beats. It begins to 

 move, and goes on swinging, because it has been 

 removed from the position in which the cord 

 hangs straight, and in which alone it can remain 

 at rest. Since gravity is the cause of the beats of 

 the pendulum, these must be faster or slower, or 



