OH the Geography/ of Plants, 269 



Algiers, which is made use of in the second comparison. Oro- 

 tava has a lower temperature than other parts of the Old World 

 in the same latitude ; partly, because it is exposed to the influence 

 of the sea, and partly, because it is situated 163 toises above 

 its level *. Algiers, on the contrary, has too high a temperature 

 from being exposed to the effects of the sandy deserts of Africa, 

 If, in the first method, we were to employ the temperature of 

 Algiers, instead of that of Oratava ; then, the mean temperature 

 of 31° 28' (5° 20' -more southward) would be 2° higher t: 

 we should thus obtain a mean temperature of 23° 1', which 

 number, compared with the mean temperature of Natchez, gives 

 a difference of 4° 9' instead of 2° o. It appears quite as impro- 

 per to estimate the mean temperature of Europe under 68° 30' 

 of latitude, according to Eirontekis ( — 2° 8') ; for Magerda, 

 the latitude of which is 71°, has a mean temperature of 

 + 0.07 X. 



This example seems sufficient to prove how uncertain and 

 fallacious it is, to institute comparisons between the two parts 

 of the world through the mean temperature of single places. 



This part of the treatise presents likewise a new verification 

 of the law, established and illustrated with so great perspicuity 

 by M. Wahlenberg§, viz., that the mean temperature of the 

 whole year affords a very inaccurate measure of vegetation. 

 Thus has Wahlenberg shewn, that Eirontekis with a mean 

 temperature of — 2° 8' has a much richer vegetation than the 

 North Cape with a mean temperature of + 0.07, or than the 

 hospital of St. Gothard with a mean temperature of — 0.9. 

 The zone also in the equatorial regions in which trees cease to 

 grow (1,800 toises above the surface of the sea) has a mean 

 temperature of nearly 9° cent. ; the elevation at which the snow- 



• Humboldt et Bonpland Reise in die aquatorialgegenden- Ister. 

 Theil, p. 450. 



t Between Rome and Algiers the author has for 6^ 5' of latitude, a dif- 

 ference of 5° 3' mean temperature. 



X Wahlenberg's FUrra Lapponica, p. 47. 



§ Ffora Lapponica; und eiue AWumdlung Uber die Temperatur der 

 Quellen ; in den Schriften der Schwedischen Gesellschafl der Wissen* 

 schaften, 1811. 



