66 THE SUN. 



of the sun's surface is equal to that which would be pro- 

 duced by burning on that square yard six tons of coal 

 per hour, and keeping up constantly to that rate of con- 

 sumption which, if used to the greatest advantage, would 

 keep a 63,000 horse steam-engine at work. And this, 

 mind, on each individual square yard of that enormous 

 surface which is 12,000 times that of the whole surface 

 of the earth ! 



(26.) Let me say something now of the light of the sun. 

 The means we have of measuring the intensity of light 

 are not nearly so exact as in the case of heat but this at 

 least we know that the most intense lights we can pro- 

 duce artificially, are as nothing compared surface for sur- 

 face with the sun. The most brilliant and beautiful light 

 which can be artificially produced is tliat of a ball of 

 quicklime kept violently hot by a flame of mixed ignited 

 oxygen and hydrogen gases playing on its surface. Such 

 a ball, if brought near enough to appear of the same size 

 as the sun does, can no more be looked at without hurt 

 than the sun but if it be held between the eye and the 

 sun, and both so enfeebled by a dark glass as to allow of 

 their being looked at together it appears as a black 

 spot on the sun or as the black outline of the moon in 

 an eclipse, seen thrown upon it. It has been ascertained 

 by experiments -which I cannot now describe, that the 

 brightness, the intrinsic splendour, of the surface of such 

 a lime-ball is only one 146th part of that of the sun's 

 surface. That is to say, that the sun gives out as much 

 light as 146 balls of quicklime each the size of the sun, and 

 each heated over all its surface in the way I have de- 



