THE SUN. 



71 



principles by which the weights of the bodies of the universe are ascertained 

 is in spirit the same for all, it may be worth while here to explain the method, 

 once for all, in its application to the sun. 



When a body revolves in a circle, we know from common and familiar ex- 

 periments that it has a tendency to fly from the centre of 'the circle, which 

 tendency is greater the more rapidly the body revolves and the greater its dis- 

 tance 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 cen- 

 trifugal 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 discov- 

 ered by a series of simple experiments which wil 1 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. 



To apply this general principle, then, to the case before us, let it be consid- 

 ered 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 exercises 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 tho moon and the centre of the earth. And 

 yet the moon is held as firmly and steadily in its circular course round the 

 earvh, 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 resist- 

 ed. 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 cir- 

 cular 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 ex- 

 plained, 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 revolving in a circle round 

 it at a known distance, and in a known time. 



While, on the one hand, we know the distance and time of the moon's revo- 

 lution round the earth, we also know the distance and time of the earth's revo- 

 lution round the sun. We are thus, allowing for the difference of the two 

 distances, in a condition to compare the actual amount of attraction which the 

 earth and the sun respectively exercise upon bodies revolving round them, and 

 we find, accordingly, that the attraction exercised by the sun upon any body 

 is greater than the attraction that would be exercised by the earth upon the 

 same body in a like position, in the proportion of three hundred and fifty thou- 

 sand to one. But as these attractions are, in fact, produced by the respective 

 masses of matter composing the sun and the earth, it follows that the weight 



