ASTRONOMY. 179 



f. Though Mercury is too much immersed in the 

 Sun's rays to allow points of unequal splendour to 

 be observed on his surface, yet certain periodical 

 inequalities, observed in the Horns of the disk, 

 seem to indicate a revolution on an axis. 



Of the other planets, Uranus is too distant, and Ju- 

 no, Vesta, Ceres and Pallas, too small, to admit 

 of any observation from which either a revolution 

 on an axis, or the contrary, can be inferred. 



177 Jupiter, Saturn and Mars, are compress- 

 ed at the poles, or the extremities of the axes 

 of revolution, in the same manner with the 

 Earth. 



a. The compression of Jupiter amounts to a four- 

 teenth part of his longer diameter ; that of Saturn 

 to an eleventh. 



b. Mars, according to the observations of Dr HER- 

 SCHEL, is compressed at the poles, so that his 

 equatorial diameter is to his polar axis as 16 to 15 

 nearly. Phil. Trans. 1781, p. 134?. and 1784, 

 p. 259. LA LANDE, 3343. 



178. Some appearances indicate the existence 

 of Atmospheres surrounding the planets Jupi- 

 ter, Mars and Venus, but are not sufficient to 

 establish the fact with perfect certainty. 



VINCE, 400, &c. BJOT, torn. m. 37, &c. 



SECT. 



