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/z/// 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. 



