THE SUN. 57 



other impulse conveyed along a steel bar, is about six- 

 teen times greater than in air. Now, suppose the sun 

 and the earth connected by a steel bar. A blow struck 

 at one end of the bar, or a pull applied to it, would not 

 be delivered would not begin to be felt at the sun till 

 after a lapse of 313 days. Even light, the speed of 

 which is such that it would travel round the globe in less 

 time than any bird takes to make a single stroke of his 

 wing, requires seven minutes and a half to reach us from 

 the sun. 



(12.) The illustration of the distance of the sun which 

 I have just mentioned, by supposing it connected with 

 the earth by a steel bar, will serve to give us some notion 

 of the wonderful connexion which that mystery of mys- 

 teries, gravitation, establishes between them. The sun 

 draws or pulls the earth towards it. We know of no 

 material way of communicating a pull to a distant object 

 more immediate, more intimate, than grappling it with 

 bonds of steel; and how such a bond would suffice we 

 have just seen. But the pull on the earth which the sun 

 makes is instantaneous, or at all events incomparably 

 more rapid in its transmission across the interval than 

 any solid connexion would produce, and even demon- 

 strably far more rapid than the propagation of light 

 itself.* 



(13.) Let me now try to convey some sort of palpable 



notion of the size of the sun itself. On a circle six feet 



in diameter, representing a section of it through the 



centre, a similar section of the earth would be about 



* See note at the end of this lecture. 



