206 QUARTERLY JOURNAL. 



cause alone, be slightly reduced. Then, again, a southern expo- 

 sure is more favorable to vegetation than a northwest exposure. 

 The color and composition of soils, too, cannot be overlooked in 

 the list of causes which modify climate. These, however, are 

 iiLore local and less general, unless, indeed, the area is greatly ex- 

 tended. 



Places which are known to enjoy an equal temperature are 

 called isothermal^ signifying merely an equal temperature. Ima- 

 ginary lines connecting such places, are called isothermal Xmes., 

 We have already said, in effect, that these cannot coincide with the 

 lines of latitude drawn upon a map, or a terrestrial globe ; thej] 

 therefore intersect them at various angles, and form by themselves 

 a peculiar system of lines approaching only to parallelism among 

 themselves — ^for example, those which are drawn upon a map ol 

 Europe, do not coincide or run parallel to those of America; 

 as the line of equal temperature in the two continents runs twelvi 

 degrees farther north in the former than in the latter. 



Another result which has been obtained by careful observatioi 

 is, that two places may receive an equal amount of heat, during j 

 part of the year only ; that is, it will enjoy an equal summer or ai 

 equal winter temperature. Lines or curves, representing thes" 

 facts, are termed isotheral and isocheimal lines ; the former refeB^I 

 ring to equal summer, the latter to equal winter temperature. ' 



The temperature of the atmosphere is influenced by, and de 



pends upon, the heating power of the earth, the rays of the su: 



passing through it without imparting their caloric directly to it 



particles. On being received, however, upon the surface of th 



earth, it becomes heated, and then imparts a portion of its tempe 



rature to the stratum of air in immediate contact with it. This, ii 



consequence of its expansion, has its specific gravity diminished 



and hence rises and gives place to another stratum, which in it 



turn also ascends. By these changes of place then, the body o 



the atmosphere is heated and its temperature elevated. But thi 



process only expends a part of the heat received from the sun 



The remaining heat is conducted downwards from particle to par 



tide, diminishing of course according to the distance from the sur 



face, till finally we reach a depth which is unaffected by the sun' 



rays. The superficial temperature of the soil of any given plac 



'will vary as well as that of the air ; the color of the soil, itsna 



