SECT. XXV. SUN'S INFLUENCE ON THE PLANETS. 225 



heat emitted by the sun. Sir William Herschel was the first to 

 suspect that it was affected by the quantity and magnitude of 

 the spots on his surface ; Professor Secchi has observed that the 

 spots are less hot than the luminous part ; and now Professor 

 Wolf has perceived that the amount of heat emitted by the sun 

 varies periodically with the spots every ll'll years, or nearly 

 nine times in a century, beginning at the commencement of the 

 present on^. He has discovered a sub-period in that of the 

 spots, which no doubt has an effect on the quantity of solar heat. 

 So the unaccountable vicissitudes in the temperature of different 

 years may ultimately be found to depend upon the constitution 

 of the sun himself. 



The intensity of the sun's light diminishes from the centre to 

 the circumference of the solar disc. His direct light has been 

 estimated to be equal to that of 5563 wax candles of moderate 

 size placed at the distance of one foot from an object ; that of the 

 moon is probably only equal to the light of one candle at the 

 distance of 12 feet : consequently the light of the sun is more 

 than three hundred thousand times greater than that of the 

 moon. According to Professor Secchi's experiments at Rome, 

 the heat of the solar image is almost twice as great at the centre 

 as at the edge. The maximum heat, however, is not in the 

 centre, but in the solar equator, and the spots are less hot than 

 the rest of the surface. 



The oceans of light and heat probably arising from electric or 

 chemical processes of immense energy that continually take place 

 at the sun's surface (N. 217) are transmitted in undulations by 

 the ethereal medium in all directions ; but notwithstanding the 

 sun's magnitude and the inconceivable intensity of light and heat 

 that must exist at his surface, as the intensity of both diminishes as 

 the square of the distance increases, his kindly influence can hardly 

 be felt at the boundaries of our system. In Uranus the sun 

 must be seen like a small brilliant star not above the hundred 

 and fiftieth part as 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 and Neptune. 



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