THE PLANETS, ARE THEY INHABITED ? 



density and less reflecting power than those which float over 

 the general atmosphere and form the ground on which the belts 

 and spots are seen. 



That the atmosphere has not any very extraordinary height 

 above the surface of the planet is proved by the sharply denned 

 edge of the disc. If its height bore any considerable proportion 

 to the diameter of the planet, the light towards the edges of the 

 disc would become gradually fainter, and the edges would be 

 nebulous and ill-defined. The reverse is the case. 



13. One of the most remarkable consequences of the rotatory 

 motion, which has been the means of giving to the inhabitants 

 of the earth the alternations of day and night, is that its figure 

 has been changed from that of a perfect sphere to an oblate 

 spheroid ; that is, a globe flattened at the poles. This has been 

 already explained. 



If the diurnal rotation of the earth were more rapid than it is, 

 this polar flattening would be more considerable. In short, the 

 degree of oblateness, or the proportion in which the polar axis is 

 shorter, than the equatorial diameter, depends on the time of 

 rotation in such a manner, that this time being known, that pro- 

 portion can be computed, or vice versd. 



Now, the rotation of these major planets being ascertained, 

 and being much more rapid than that of the earth, it would 

 follow that they must be oblate spheroids, and that their degree 

 of oblateness must be much greater than that of the earth. 

 Observation fully confirms this. 



The disc of Jupiter, seen with magnifying powers as low as 30, 

 is evidently oval, the lesser axis of the ellipse coinciding with 

 the axis of rotation, and being perpendicular to the general 

 direction of the belts ; as in the case of the earth, the degree of 

 oblateness of Jupiter is found to be that which would be produced 

 upon a globe of the same magnitude, having a rotation such 

 as the planet is observed to have. 



At the mean distance from the earth, the apparent diameters 

 of the disc are ascertained by exact micrometric measures 

 to be 



Miles. 



Equatorial Diameter . . . . . 38 '4 "=92080 

 Polar Diameter , 35'6" = 85210 



Mean Diameter . . . =88645 



The polar diameter is therefore less than the equatorial, in the 

 ratio of 356 to 384, or 100 to 108 nearly. Other estimates give 

 the ratio as 100 1o 10^ 



42 



