THE PLANETS, ARE THEY INHABITED"? 



19. .Question of habitability of these planets considered. in. reference 

 to suu's light. and heat. 20. Great comparative magnitude of these 

 planets Their volumes Question of habitability continued. 21. 

 Proportionate population, if inhabited. 22. Investigation of physical 

 causes incompatible with their being habitable globes. 23. Application, 

 of such causes to Jupiter, and reasoning thereon Necessity of organised 

 world being different from that on the earth. 24. Comparative volume 

 and density of the Earth and Jupiter. 25. Comparison of relative 

 quantities of gravitating matter in Saturn, Uranus, and Neptune, and 

 in the Earth and of density. 26. Comparative weights of bodies 

 placed upon such planets and on the Earth. 27. General results of 

 inquiry as to the habitability of these planets. 28. Atmosphere of 

 these planets. 29. Their diurnal revolution General observations on 

 rotation and their results Position of axis of rotation. 



1. IN the absence of an atmosphere we could have no. clouds ; 

 day would be one unvaried wearisome glare of the sun. The 

 bright azure sky, so grateful to the sight, is nothing more than 

 the natural colour of the air reflected to the eye. The air which 

 fills a room is not perceived to be blue only because it is not 

 present in sufficient quantity to excite in the eye any perception 

 of its colour ; just as a glass of sea- water seems translucent 

 and colourless, while the same water viewed through a consider- 

 able depth, appears with its proper hue of green. 



When we look up, therefore, through forty miles of air, we 

 behold that fluid of its proper tint of blue. In the absence of the 

 atmosphere the great vault of the heavens would present one 

 unvaried and eternal black, the stars dimly twinkling here .and 

 there, the whole forming a most funereal contrast with the bright 

 orb which would be seen holding its solitary course through this 

 eternal expanse of darkness. 



2. The atmosphere produces effects on the temperature of our 

 habitation which are not less important. It retains and diffuses 

 warmth, whether proceeding from the sun above, or from sources 

 of internal heat within the globe itself. What situation with 

 respect to temperature we should be placed in by its absence, 

 or even by a considerable diminution of its quantity or density, 

 may be easily inferred by considering the state of those parts of 

 the earth which are placed at such an altitude as to leave below 

 them a large portion of the atmosphere. The summits of lofty 

 ridges, such as those of the Alps, the Andes, and the Himalayan, 

 are examples of this. No intensity of direct solar heat can com- 

 pensate for the absence of a sufliciently dense atmosphere, and 

 even within the tropics, water cannot exist in a liquid form at 

 elevations above 14,000 feet. The summits of the Andes are 

 clothed in everlasting snow. 



Had we, therefore, been unprovided with an atmosphere, or 



