142 LIGHT SCIENCE FOR LEISURE HOURS. 



tilt. One can understand at least the possibility that 

 some peculiarity in the atmosphere of the planet 

 might serve to remedy the effects of the former cir- 

 cumstance ; precisely as our English climate is tempered 

 by the abundant moisture with which the air is ordi- 

 narily laden. But while we can conceive that the 

 minute and almost starlike sun of the Uranian skies 

 may supply much more heat than its mere dimensions 

 would lead us to expect, it is difficult indeed to under- 

 stand how the absence of that sun for years from the 

 Uranian sky can be adequately compensated. Yet in 

 Uranian latitudes corresponding to the latitude of 

 London the sun remains below the horizon for about 

 twenty-three of our years in succession. Such is the 

 Arctic * night of regions in Uranus occupying a posi- 

 tion corresponding to that of places in our temperate 

 zone. 



But the most important result of the discovery of 

 the satellites has been the determination of the mass 

 or weight of the planet, whence also the mean density 

 of its substance has been ascertained. It has been thus 

 discovered that, like Jupiter and Saturn, Uranus is con- 

 structed of much lighter materials than the earth. Our 

 earth would outweigh almost exactly six times a globe as 

 large as the earth but no denser than Uranus. It is to be 



1 It has been remarked that there is some incongruity in the name 

 Arctic planets which I have assigned in my ' Other Worlds ' to Uranus 

 and Neptune, when considered with reference to the theory I have enun- 

 ciated that these planets still retain an enormous amount of inherent 

 heat. Many seem to imagine that the term arctic implies cold. I have, 

 of course, only used the name as indicating the distance of Uranus and 

 Neptune from the sun. 



