SECT. XXVI. CONSTITUTION OF THE SUN. 239 



be surrounded by a mottled ocean of flame, through 

 which his dark nucleus appears like black spots often of 

 enormous size. These spots are almost always com- 

 prised within a zone of the sun's surface, whose breadth, 

 measured on a solar meridian, does not extend beyond 30$ 

 on each side of his equator, though they have been seen 

 at the distance of 39i. From their extensive and rapid 

 changes, there is every reason to suppose that the exte- 

 rior and incandescent part of the sun is gaseous. The 

 solar rays, probably arising from chemical processes that 

 continually take place at his surface, or from electricity, 

 are transmitted through space in all directions ; but not- 

 withstanding the sun's magnitude, and the inconceivable 

 heat that must exist at his surface, as the intensity both 

 of his light and heat diminishes as the square of the dis- 

 tance increases, his kindly influence can hardly be felt 

 at the boundaries of our system, or at all events it must 

 be but feeble. 



The direct light of the sun has been estimated to be 

 equal to that of 5563 wax candles of moderate size, sup- 

 posed to be placed at the distance of one foot from the 

 object. That of the moon is probably only equal to the 

 light of one candle at the distance of twelve feet. Con- 

 sequently the light of the sun is more than three hundred 

 thousand times greater than that of the moon. Hence 

 the light of the moon imparts no heat. Professor Forbes 

 is convinced by recent experiments that the direct light 

 of the moon is incapable of raising a thermometer one 

 three-hundred-thousandth part of a centigrade degree, 

 at least in this climate. The intensity of the sun's light 

 diminishes from the center to the circumference of the 

 solar disc. 



In Uranus, the sun must be seen like a small but bril- 

 liant star, not above the hundred and fiftieth part so 

 bright as he appears to us ; but that is 2000 times brighter 

 than our moon ; so that he is really a sun to Uranus, 

 and may impart some degree of warmth. But if we 

 consider that water would not remain fluid in any part 

 of Mars, even at his equator, and that in the temperate 

 zones of the same planet even alcohol and quicksilver 

 would freeze, we may form some idea of the cold that 

 must reign in Uranus. 



