with Geographic Botany. — Temperature. 315 



tudes are also found to have a smaller range of depression 

 below the air^ and the maximum in the tropics is perhaps not 

 far from 12°. As the subject expands desirable observations 

 rapidly become scarce, and though many reasons lead us to 

 believe that terrestrial radiation increases on elevation, a soli- 

 tary experiment alone supports it. Among a few observa- 

 tions at the mountain-station in Jamaica, already mentioned, 

 one has a depression of 18^. From these statements Mr. 

 Daniell is led to infer, " that the same cause which obstructs 

 the passage of radiant - heat in the atmosphere from the sun, 

 opposes also its transmission from the earth into space.^^ 



Latitude then cannot be refused the first station in the dif- 

 fusion of heat ; as it is increased, or as the path of the sun is 

 distanced on the surface of the earth, temperature progress- 

 ively decreases. Such is the general feature of its distribu- 

 tion; but every spot possesses a number of circumstances 

 continually active in modifying it. These vary so much in 

 different places, that it becomes necessary, in estimating the 

 temperature of any one place, to take an assemblage of cir- 

 cumstances into consideration which perhaps hardly occur 

 in any other. Europe naturally becomes with us a standard 

 for comparison as to climate with other portions of the globe ; 

 but Europe is situated among a union of favourable influ- 

 ences, which render its climate milder than that of any other 

 large surface of land : hence deductions made from it will be 

 too favourable. Besides, from the mildness of the European 

 climate, errors are daily made as to the qualities of others ; 

 they are hastily condemned as severe and extreme, when in 

 all probability only a fair mean of the general climates of the 

 earth. Comparisons of this kind will establish no similarity ; 

 their chief value and importance consists in eliciting facts. 

 Resemblances have long been sought between the northern 

 and southern hemispheres, but every inquiry has only added 

 fresh proofs that a different distribution of temperature takes 

 place, such as might be expected from the relations of land 

 and water, elevated lands, and other minor causes. The mean 

 annual heat also does not explain what these are, nor the 

 range of their influence ; a deeper search is necessary to ob- 

 tain only a small acquaintance with them. 



Differences have been traced between the diffusion of heat 

 in the old and new aa orld. North America is a country sub- 

 ject to a climate of extremes ; it has been described as com- 

 bining a tropic summer with an arctic winter. The distri- 

 bution of its heat is very different to that experienced in Eu- 

 rope ; an estimate from the thirtieth to the sixtieth parallels 

 gives for every ten degrees the relative proportions of 3, 9, 



Y2 



