WILLIAMSON ON COMETS. 191 



any indications of an atmosphere. From this it is clear that his atmos- 

 phere is very rare, as it should be, according to the above hypothesis. 



Mars is the next planet in order, and he is provided with a very 

 considerable weight of atmosphere. When he passes near a fixed star, 

 his atmosphere has been observed to hide the star, although the body 

 of the planet did not interfere. The atmosphere of Mars is computed 

 to be equal to two thirds of his diameter. This implies an atmosphere 

 greatly superior to the atmosphere of this globe, and so it should be, 

 since he is at least forty-nine millions of miles farther from the sun. 



Of the atmospheres of Jupiter and Saturn we have little to say with 

 any degree of certainty, their distance being so great. But we observe 

 that the belts, or belt-like appearance of both those planets, exhibit 

 such frequent variations in shape and situation as cannot be accounted 

 for or explained except by the supposition of their having an atmos- 

 phere, and that a great one, often suspending large clouds. 



We cannot discover any reason why all the planets should thus be 

 provided with atmospheres, unless it was that they might all be habit- 

 able; but those planets claim our attention on another account. The 

 axis of every planet, as far as we can discover, is inclined to the plain 

 of its orbit. The effect of such inclination, like that of the earth, is a 

 change of seasons ; a change that seems to be calculated to increase the 

 fruits of the soil, for the benefit of man and beast. From the proofs 

 already stated, and this additional circumstance, we infer, without hesi- 

 tation, that the planets are all inhabited. 



I have said nothing concerning the four new planets that have been 

 discovered within the last century. They are small, and their dis- 

 tance is great — between two and three hundred millions of miles. But, 

 from the thick coma with which they are surrounded, it is obvious that 

 they have a weighty atmosphere. 



