THE PLANETS, ARE THEY INHABITED ? 



probable that with improved powers of that instrument, like 

 -appearances will be observed upon them. 



29. It might be imagined that the circumstance of being thus 

 constantly eirveloped in clouds would render it impossible to 

 ascertain whether these planets, like the earth, turn upon an 

 -axis, and consequently have days and nights analogous to those 

 of the earth. 



It must be remembered, however, that the earth's atmosphere, 

 and the clouds which float upon it, partake of the motion of 

 diurnal rotation, and that if the atmosphere were perfectly calm 

 for twenty-four hours, the various masses of clouds resting upon 

 it being always suspended over the same parts of its surface, 

 would be carried round with it, and would consequently make a 

 complete rotation round the common axis in the same time 

 exactly as the solid globe of the earth. 



Now, if an observer placed upon any of the planets, not too 

 remote, were in this case to direct a sufficiently powerful telescope 

 to the earth, although he might not see the outlines of land and 

 water, being enveloped by clouds, he would distinguish the 

 masses of clouds themselves by their varieties of light and shade, 

 -dud would see them carried round by the diurnal rotation dis- 

 appearing at one side and reappearing at the other ; and he 

 would thus not only ascertain the fact of the diurnal rotation of 

 the earth, but also the time of rotation, that being the interval 

 between two successive disappearances or reappearances of the 

 same lineaments of light and shade, anil the direction of the 

 axis of rotation that direction being at right angles to the 

 apparent motion of rotation. 



Circumstances, just such as these, have been observed to take 

 place on Jupiter and Saturn. The masses of cloud, whose 

 lights and shadows diversify their surfaces, though more or less 

 shifting and variable, are at times found to remain fixed, as if the 

 atmosphere were absolutely calm and quiescent for intervals 

 sufficiently protracted to enable the telescopic observer to see 

 the same lineaments disappear at one side of the disk, reappear 

 at the other, and passing across the disk, again disappear. These 

 are the obvious effects of the rotation of these planets upon an 

 axis at right angles to the direction of this apparent motion. 



Observations such as these, repeated and continued for long 

 periods of time, have led to the discovery that Jupiter turns upon 

 a certain diameter with a diurnal motion, making a complete 

 revolution in 9 h 55 m 26' terrestrial time, and that Saturn 

 revolves in like manner, and what is more remarkable in a time 

 not very different from that of the rotation of Jupiter. Saturn's 

 rotation is completed ill 10 h 29'" 17 s . 

 32 



