496 Photographs of Jupiter taken at the Lowell Observatory . 



at their sides, where they leave the belts, and tenuous in 

 mid-career. What they betoken is difficult to decide, 

 because from their dark tint they would appear of a negative 

 rather than of a positive character, gaps, that is, in the 

 cloud envelop, rather than entities in themselves. That they 

 give us a clue to the meteorological circulation of the planet 

 is clear ; but it is not at all clear what that clue is. That 

 they are an essential feature of a planet in the early semi- 

 chaotic state is indicated by a recent discovery at this obser- 

 vatorv : that Saturn's belt is laced, though much more 

 faintly, in precisely the same manner. 



5. The colour of the dark markings of Jupiter is everywhere 

 a cherry-red. As the physics of his condition show that he 

 must be very hot, his density not being compatible with his 

 gravity under any other supposition, the dark markings 

 must be where the lower and hotter layers of the body show 

 through. For that red heat is the maximum possible for 

 his surface glow is evidenced by his albedo *78 of pure 

 reflexion ; no perceptible light being emitted by him. We 

 are thus warranted in concluding that the lighter, yellowish 

 belts lie at a higher altitude, and are what they seem to 

 be, clouds. From Dr. Slipher's spectrograms these clouds 

 appear to be made up in part at least of water-vapour, in 

 part probably of other substance, though about this we have 

 no positive evidence, as the spectroscope informs us only that 

 unknown gases are certainly present in the planet's atmo- 

 sphere, but gives no assurance that these are condensed to 

 cloud, possible and even probable as this may be. That the 

 bright belts are cloud and show no self-emission of light 

 appears also from their albedo being almost exactly that of 

 cloud, 0-78 as against 0'72. 



The various lines of evidence thus point to one conclusion : 

 that Jupiter is still in a fiery chaotic state, that the materials 

 that went to make him are still in disconnected fluid form, that 

 they are much condensed toward his centre, but are kept from 

 absolute solidification by his intense heat; in short, that he 

 is in a midway stage between Sun and Earth. 



6. To the great use to which these photographs and their 

 successors may be put I may here draw attention. Per- 

 mitting, as they do, of measurement of the planet's features 

 it will be possible in the future to get a record of alterations 

 in the appearance of the belts which no one may dispute 

 and any one may verify by inspection himself of reprints 

 from the negatives, and which in addition allow of quanti- 

 tative valuation. Thus the rotation time of the several belts 



