108 THE BORDERLAND OF SCIENCE. 



adding only to the brightness of the central parts of 

 the disc. I would not insist too strongly on this 

 inference, because the darkening due to oblique 

 incidence is, under certain circumstances, very obvious 

 to direct observation. But it seems to us that a 

 portion of the difference should be referred to the 

 inherent luminosity of the central parts of the disc. 

 This being admitted, it would follow that the real 

 solid globe of the planet is much smaller than the 

 globe measured by astronomers ; and that, therefore, 

 instead of that amazingly small density which is so 

 perplexing a feature of the planet's physical condition, 

 Jupiter's globe may have a density equalling or 

 exceeding that of the earth. 



And after all, let us remember that the theory that 

 Jupiter is an intensely heated globe a theory to 

 which we have been led by the consideration of many 

 observed facts, and which in its turn suggests very 

 satisfactory explanations of other observed facts 

 would merely show that, as Jupiter and Saturn hold an 

 intermediate position between the sun and the minor 

 planets in respect of size, so those giant orbs hold a 

 corresponding position in respect of inherent heat. 

 Koughly speaking, the earth is 8,000 miles, the sun 

 840,000 miles, in diameter, and Jupiter, with his 

 diameter of 82,000 miles, comes midway between these 

 orbs. Now, the sun is at a white heat, and the earth 

 gives out only what is called obscure heat; and if 

 Jupiter's globe is at a red heat, he again comes mid- 

 way between the sun and the earth. 



