Temperature of the Globe. 3S0 



under the equator, where the mean temperature rises to 82" 

 Fahr. (22°.2 R.) it is not much warmer than it is in 10^ 

 north and south latitude. According to the Commentary of 

 Geminus on the Astronomical Poem of Aratus *, some Greek 

 philosophers believed the temperature of the tropics even to sur- 

 pass that of the equator. M. Arago haS;, in a very ingenious 

 manner, demonstrated, by numerous optical experiments, that, 

 from the vertical incidence to a zenith distance of 20°, the quan- 

 tity of the reflected light (and the lesser heating of the illumi- 

 nated body depends on this quantity), remains almost the 

 same. In comparing the mean annual temperatures with one 

 another, I find, that, in the western part of the old continent, the 

 temperatures diminish from the south towards the north in the 

 following proportion -f* : 



From 20° to 30° north Latitude. 3°.2 Reaum. 



30 40 3.6 « 



40 50 5 .7 



50 60 4.4 



In both the continents, the region where the diminution of heat 

 is most rapid, is to be found between 40° and 45° latitude. In 

 this result, the observation agrees in a remarkable manner with 

 the theory ; for the variation of the square of the cosines which 

 expresses the law of the mean temperature, is largest at 45° lati- 

 tude. This circumstance, as I have shown in another place, has 

 exercised a very beneficial influence on the state of civilisation 

 of those nations who live in the mild countries, under this, the 

 medium parallel of latitude. There the district where the vine 

 grows, borders upon that of the olive and orange tree. No- 

 where else upon earth (in proceeding from the north to the 

 south) does the heat increase more rapidly with the geographi- 

 cal latitude ; nowhere else do the various vegetable productions, 

 used in gardening and in agriculture, succeed each other more 



• Esig. in Aratum eays. 13. Strabo, Geogr. lib. ii. p. 97* 

 + In the eastern parts of the new continent, the diminutions of the mean 

 temperature are as follows : 



Trom 20" to 30° Latitude, 5° Reaum. 

 30 40 5.7 



40 50 72 



50 60 5.8 



