124 COSMOS. 



inhabitability of the planet in particular zones. According 

 to Plutarch, De Plac. Philos., ii., 8, Anaxagoras believed 

 •' that the world, after it had come into existence and pro* 

 duced from its womb living beings, had of itself inclined to- 

 ward the south." In the same regard, Diogenes Laertius 

 says of the Clazomenier, " the stars had originally projected 

 themselves in a dome-like layer, so that the pole appearing 

 at any time was vertically over the Earth ; but that after- 

 ward they assumed an oblique direction." The origin of the 

 obliquity of the ecliptic was considered as a cosmical event. 

 There was no question respecting a subsequent progressive 

 alteration. 



The description of the two extreme, therefore opposite, con 

 ditions to which the planets Uranus and Jupiter approximate 

 most closely, is suited to call to mind the variations which the 

 increasing or decreasing obliquity of the ecliptic would pro- 

 duce in the meteorological relations of our planet, if these va- 

 riations were not comprised within very narrow limits. The 

 knowledge of these limits, the subject of the great works of 

 Leonhard Euler, Lagrange, and Laplace, may be called one 

 of the most brilliant achievements of modern times in theo- 

 retical astronomy and the perfected higher analysis. These 

 limits are so narrow, that Laplace {Expos, du Systeme du 

 Monde, ed. 1824, p. 303) puts forward the opinion that the 

 obhquity of the ecliptic oscillates about its mean position only 

 1^° toward both sides. According to this statement,^ the 

 tropical zone (the tropic of Cancer, as its northernmost and 

 outermost boundary) would approach only so much nearer to 

 us. The result would therefore be, if the numerous other 

 meteorological perturbations are omitted, as if Berlin were 

 gradually displaced from it present isothermal line to that 

 of Prague. The elevation of the mean annual temperature 

 would scarcely amount to more than one degree of the cen- 

 tigrade {j\ of a degree of Fahrenheit's) thermometer.! Biot, 



* " L'etendue entiere de cette variation serait d'environ 12 dcgres, 

 mais I'action du Soleil et de la Lune la reduit k peu pres k trois degr^s 

 (ceut^simaux)." " The entire extent of that variation would be about 

 12^, but the action of the Sun and Moon reduce it to very nearly 3*^ 

 (centesimal)." — Laplace, Expos, du Syst. du Monde, p. 303. 



t I have shown in another place, by comparison of numerous mean 

 annual temperatures, that in Europe, from the North Cape to Palermo, 

 the difference of one degree of geographical latitude very nearly cor- 

 responds to 0-5° of the centigrade thermometer, but in the western 

 temperature-system of America (between Boston and Charlestown) to 

 0-9O. {Asie Centrale, tom. iii., p. 229.) 



