MERCURY VENUS THE EARTH. 



75 



that it will at length cease, and an increase of the obliquity set in, to decrease again and 

 increase, oscillating about a mean position within very restricted limits. 



One of the most important attributes of our globe as a planet is the gaseous envelope 

 that encloses it on all sides the atmosphere essential to the vitality of its organised 

 occupants, man, brute, bird, and plant, but chiefly interesting to the astronomer 



from its influence in displacing celestial phenomena by 



:\Vvv;,yv:::::; ::..'.:.,', ^ ts refractive power, and diffusing the rays of light so 



. . as to surround us on every hand with visible glories. 



The atmosphere rapidly diminishes in density as we 

 $&-'/:\ recede from the surface. The diagram exhibits various 

 ifi^:;-:- strata of air resting upon the earth, the upper pressing 

 upon the lower, and causing the interior to be more 

 dense than the exterior strata. It is not known how 

 high this elastic medium extends, but 



'.-,' . " to breathe 



The difficult air on the ic'd-mountain's top," 



is an experiment which shows the atmosphere to be exceedingly rare at no greater height 

 than what is reached by many of the superficial elevations of the earth. M. Biot and Gay 

 Lussac ascended in a balloon nearly five miles the greatest altitude ever reached by 

 man. Visible clouds, however, are supposed to be sometimes twice that height, and 

 atmospheric phenomena have been noticed, conceived to have had an elevation of forty 

 miles. But the chief peculiarity of our planet is the presence of a secondary body as a 

 satellite, reflecting to its surface the light of the sun in the absence of his direct rays. 

 It has indeed been supposed that Venus is similarly dignified, and we are not in 

 circumstances to say positively that this is not the case, either in relation to her or 

 Mercury, as such an attendant, if small, might exist, and have hitherto escaped notice, 

 owing to the position occupied by those bodies. Cassini and others have imagined they 

 perceived a satellite attending Venus, but the observation has not been verified. 



The terrestrial world occupies a favoured place in the system, a position from which 

 nearly all the sister planets are visible to the naked eye. It will not be seen itself by the 

 inhabitants of Uranus, and be scarcely perceptible to those of Saturn. As an interior planet 

 to Jupiter and Mars, it will appear occasionally as a spot upon the sun's disk, performing 

 similar transits to those of Venus and Mercury. Its great divisions of land and water, 

 the outlines of its continents and seas, with its masses of ice and snow at the poles, will 

 be seen from Mars ; and at the time of the inferior conjunction of Venus, when she is not 

 more than twenty-six millions of miles removed from us, our globe will exhibit a full orb, 

 shining with great splendour through the whole of her night. 



