THE PLANETS. 453 



be called one of the most brilliant achievements Df modern 

 times in theoretical 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 obliquity of the ecliptic oscillates about its 

 mean position only 1^ towards both sides. According to 

 this statement, 88 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 its present isothermal 

 line to that of Prague. The elevation of the mean annual 

 temperature would scarcely amount to more than one degree 

 of the centigrade (^ of a degree of Fahrenheit's) ther- 

 mometer. 38 Biot, indeed, also assumes only narrow limits 

 for the alternating variation in the obliquity of the eclip- 

 tic, but considers it more advisable not to assign to it a 

 determinate number. "La diminution lente et seculaire 

 de 1'obliquite de 1'ecliptique," says he, "offre des etats 

 alternatifs qui produisent une oscillation eternelle, com- 

 prise entre des limites fixes. La theorie n'a pas encore pu 

 parvenir a determiner ces limites ; mais d'apres la constitu- 



30 " L'etendue entiere de cette variation serait d'environ 

 12 degres, mais I'action du Soleil et de la Lune la reduit a peu 

 pres a trois degres (centesimaux)." "The entire extent 

 of that variation would be about 12 degrees, but the action 

 of the Sun and Moon reduce it to veiy nearly 3 degrees 

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



39 I have shown in another place, by comparison of nume- 

 rous mean annual temperatures, that in Europe, from the 

 North Cape to Palermo, the difference of one degree of 

 geographical latitude very nearly corresponds to 0*5 of the 

 centigrade thermometer, but in the western temperature, 

 system of America (between Boston and Churlestown) to 

 0-9. (Asie Centrale, torn. iii. p. 229.) 

 n 2 



