178 ASTRONOMICAL HISTORY. 



of vapours more frequently. And if any one forms a judg 

 ment from the regularity of the heavenly bodies, and the 

 equability of the motion itself, that the heaven is immutable, 

 and should set down the exactness of their periods and re 

 newals as a distinct mark of their eternity, seeing con 

 stancy of motion seems scarcely compatible with a perish 

 able substance, he ought to advert a little more attentively 

 to this, that such a periodical reappearance, as if in a 

 cycle, at stated intervals, may be even found among our 

 selves in several things, particularly in the tides of the 

 ocean ; besides, smaller variations which may obtain among 

 the heavenly bodies, their dates and renewals, escape our 

 vision, and baffle our calculations. Nor ought the revolu 

 tion of the heavenly bodies in a circle to be taken as a 

 better proof of their eternity, because, forsooth, there is no 

 end to impulsion in a circle, and an immortality of motion 

 is agreeable to an immortality of substance. For even the 

 secondary comets, which have their place lower than the 

 moon, perform revolutions, and that from a property of 

 their own ; unless we are to give credit to the figment of 

 their being attached to stars. For if we will argue the 

 eternity of the heavenly bodies from their circular motion, 

 that ought to be referred to the entire system of the heavens, 

 not to its parts ; for the atmosphere, the sea, and the land, 

 are eternal in the totality, perishable in the parts. Besides, 

 on the contrary, we ought not to presume the motion of 

 the heavenly bodies from their revolving motion, because 

 that very motion is not a perfect one, nor renewing itself 

 absolutely in an exact and pure circle, but with declina 

 tions, eccentricities, and ellipses. Moreover, if any one 

 should retort upon us the observation we made respecting 

 the earth, in stating that those changes which occurred in 

 it were accidental not substantial, and arose from the ac 

 tion upon it of the heavenly bodies, and should assert that 

 the relation of heaven is directly contrary, since the heaven 

 can in no way be reciprocally influenced by the earth, and 

 any efflux from the earth falls short of the sphere of the 

 heavenly bodies ; so that it is probable that heaven, placed 

 aloof from all action adverse to its durability, is susceptible 

 of eternity of duration, since it is not at all agitated or in 

 fringed upon by an antagonist substance ; this seems an 

 objection not to be despised. For we are not likely per 

 sons to defer to the childish notion of Thales, who con 

 ceived that the heavenly bodies absorbed the exhalations 

 raised by the earth and sea, and were therewith fed 



