314 Thoughts on the fufpofed Varlafions 



the equator; the fun, of courfe, be continually in the equa- 

 tor; and the axisof llie earth acquire a perpendicular direc- 

 tion,; bv which means the cold of the polar regions and in 

 the temperate zones will become greater, and conlequently 

 the fitnefs of the earth for being inhabited, as well as its cul- 

 ture and fcrtilit\', niuil decreale. 



But La .Grange and La Place have freed us from any un- 

 eafinefs on this head, as they have proved, bv ingenious cal- 

 culations and conclufions, that this Anail change in the in- 

 clination of the earth's axis is an effeA of the mutual attrac- 

 tion of the planets on the orbit oi our earth; that fince the 

 time of Hipparchus it has contributed in an uniform manner 

 to the decreafe of the obliquity of the ecliptic, or the Increafe 

 of the above inclination ; that it does not always continue ; 

 but that, properlv, the above fuppofnd axis of the earth's ro- 

 tation perpendicular to the ecliptic, in confequence of this 

 action of the planets, moves in very long periods around the 

 poles of the orbit of the earth which lie in the neighbourhood 

 of the poles of the ecliptic. 



According to thefe obfervations, the variation in the obli- 

 quity of the ecliptic is merely a nutation of the earth's axis, 

 which, iince the time of Hipparchus, has occafioned a very 

 fmall decreafe in it, but will afterwards become ftationary, 

 and then produce an increafe. 



Schubert, of Peterfourgh, has found, by I-a Grange's for- 

 mula, that the obliquity of the ecliptic in the period of 65,000 

 vears always remains between 20' 43' and 27^ 45^. At pre- 

 sent it is about 43 minutes Icfs than the mean of the above 

 two quantities, and will Rill decreafe, for 4900 years, to 32" 

 53'; after which it will again increafe. 



Hence may be deduced very iniportant confequences in - 

 reo-ard to the durable Itate of our earth, on which dillant bo- 

 dies, in the courfe of its annual revolution, exercife an action; 

 but thefe bodies, on account of their fituation and great di- 

 ftance, are able to produce only very fmall and periodical nu- 

 tations, by which the maintenance of the whole is promoted, 

 and the earth i)rcl'erved frum great and difaftrous changes. 

 As long as the prcfent conflltution of the folar fyllem remains 

 the fame, thefe fmall iind harmlefs periodical nutations will 

 take place. 



But if the earth's axis and its poles are fecurcd by fuch 

 powerful bonds from all fudden or prooreilive changes, thofe 

 apparent changes aiid revolutions which wc obicrve at and 

 under the furface of our earth mull be referred to other 

 cr.ules than a change in the earth's axis, or difplarement of 

 its poles ; and this will be Hill more evident when it is con- 



liilereU 



