58 THE SUN. 



represented by a fourpenny-piece, and a distance of a 

 thousand miles by a line of less than one-twelfth of an 

 inch in length. A circle concentric with it, representing 

 on the same scale the size of the moon's orbit about the 

 earth, would have for its diameter only thirty-nine inches 

 and a quarter, or very little more than half the sun's. 

 Imagine, now, if you can, a globe concentric with this 

 earth on which we stand ; large enough not only to fill 

 the whole orbit of the moon, but to project beyond it on 

 all sides into space almost as far again on the outside ! 

 A spangle, representing the moon, placed on the circum- 

 ference of its orbit so represented, would require to be 

 only a sixth part of an inch in diameter. 



(14.) It is nothing to have the size of a giant without 

 the strength of one. The sun retains the planets in their 

 several orbits by a powerful mechanical force, .precisely 

 as the hand of a slinger retains the stone which he whirls 

 round till the proper moment comes for letting it go. 

 The stone pulls at the string one way, the controlling 

 hand at the centre of its circle the other. Were the 

 string too weak, it would break, and the stone, prema- 

 turely released, would fly off in a tangential direction. If a 

 mechanist were told the weight of the stone (say a pound), 

 the length of the string (say a yard, including the motion 

 of the hand), and the number of turns made by the stone 

 in a certain time (say sixty in a minute, or one in a 

 second), he would be able to tell precisely what ought to 

 be the strength of the string so zsjust not to break; that 

 is to say, what weight it ought at least to be able to lift 

 without breaking. In the case I have mentioned, it 



