GRAVITATION. 



and familiar experiments that it has a tendency to fly from the 

 centre, which tendency is greater the more rapidly the body 

 revolves and the greater its distance from the centre. The boy 

 who whirls a stone in a sling is conscious of this physical truth. 

 The stone, as it revolves, stretches the string with a certain 

 definite force ; this force is not in the gravity of the stone, for it 

 would be equally manifested if the stone revolved in a horizontal 

 plane. It is that tendency which we have just adverted to, and 

 which is technically called centrifugal force. If you increase the 

 velocity with which the stone is whirled round, you will find the 

 string will be more and more tightly stretched, and you may 

 augment the velocity to such an extent as to break the string. 

 If you lengthen or shorten the string, preserving the same velocity 

 of rotation, you will find that the tendency to stretch the string 

 will be proportionally increased or diminished ; in short, a fixed 

 rule or law, as it is called, will be easily discovered by a series of 

 simple experiments which will enable us to predict how much the 

 string will be stretched, provided we know the distance of the 

 revolving weight from the centre of the circle and the time it takes 

 to make each revolution. 



8. To apply this general principle, then, to the case before us, 

 let it be considered that the moon in its monthly course revolves in 

 a circle round the centre of the earth. "We know its distance, and 

 we know the time which it takes to make each revolution, we are 

 therefore in a condition to declare with what force it would stretch a 

 string, tying it to the centre of the earth. That the moon exer- 

 cises such a force cannot then be doubted. But on what, it will 

 be asked, is that force expended ? There is no string, rod, or any 

 other material or tangible connection between the moon and the 

 centre of the earth. And yet the moon is held as firmly and 

 steadily in its circular cours j round the earth, as if it were tied to 

 the centre by a string. In the absence of the string there must 

 then be some physical agency which plays its part ; there must be 

 something to resist that tendency which the string, if there, would 

 have resisted. That something was discovered by Newton to be 

 the attraction of the earth's GRAVITATION exercised upon the 

 moon and holding the moon in its circular orbit, in the same 

 manner that it would be held by the string which has been just 

 described. As we know, by the simple mechanical law above 

 explained, the force with which that string would be stretched by 

 the moon in this case, we are enabled by the same principle to say 

 what is the amount of attractive force which the earth exercises 

 upon the moon to keep it in its monthly orbit. 



In this manner, in general, we are enabled to estimate the force 

 of attraction which a central mass exercises upon another body 



103 



