5<5 THE SUN. 



length. The necessary observations were made at the 

 time of the last " transit" in 1769, and will no doubt be 

 repeated on the next occasion of the same kind, in 1874.* 



(10.) From the distance of the sun so obtained, and 

 from its apparent size (or, as astronomers call it, its 

 angular diameter), measured very nicely by delicate 

 instruments called micrometers, the real diameter of the 

 sun has been calculated at 882,000 miles, which I sup- 

 pose may be taken as exact to a few odd thousands. 



(u.) Now, only let us pause a little, and consider 

 among what sort of magnitudes we are landed. It runs 

 glibly over the tongue to talk of a distance of 95,000,000 

 of miles, and a globe of 880,000 miles in diameter, but 

 such numbers hardly convey any distinct notion to 

 the mind. Let us see what kind of conception we can 

 get of them in other ways. And first then, as to the 

 distance. By railway, at an average rate of 40 miles an 

 hour one might travel round the world in 26 days and 

 nights. At the same rate it would take 270 years and 

 more to get to the sun. ' The ball of an Armstrong 100- 

 pounder leaves the gun with a speed of about 400 yards 

 per second. Well, at the same rate of transit it would 

 be more than thirteen years and a quarter in its journey 

 to reach the sun ; ; and the sound of the explosion (sup- 

 posing it conveyed through the interval with the same 

 speed that sound travels in our air), would not arrive 

 till half a year later. The velocity of sound, or of any 



* The distance above stated is that which results from this more 

 precise mode of procedure. See this explained in Lecture V., 

 17- 



