236 RESULTS OF COMPARATIVE OBSERVATIONS: BAVARIA. 



that densely-wooded regions are warmer than where the growth is thin.^ 

 In mild winter days, with a warm southwest wind, the forest is colder 

 than the clearings in the day-time. 



Comparison of the air-temperature and soil-temperature at different seasons. 



In the clearings, the mean temperature of the air in summer and 

 winter were almost the same as the temperature of the surface of the 

 soil. Comparatively small variations occurred in the spring and fall, 

 when in the total average the surface of the soil was 0.° 03 (It) colder 

 (in spring) — or 0.° 37 warmer (in autumn) than the air. 



Amount of moisture contained in the air in the forest and the openfeldj or 

 influence of the forest on the amount of icater contained in the air. 



The climate of any place is determined by the temperature and moist- 

 ure of the air. Watery vapor forms one of the most important constitu- 

 ents of the atmosphere. It is often observed condensed in visible form 

 in small vesicles as fog or cloud, and it is always present in greater or less 

 amount as invisible gas, even in a perfectly clear sky. As this atmos- 

 pheric vapor in several respects is of great importance, the determina- 

 tion of its amount at different times and places is one of the most impor- 

 tant tasks in meteorological researches. Without vapor there would be 

 no clouds, no rain, fog, dew, snow, frost, or bail, no lightning or thunder, 

 no rainbow, no blue sky, no rosy twilight. We will state here only what 

 is most essential regarding the importance of moisture in the air. 



Damp air deadens the extremes of heat and cold. Where the air is 

 dry, we are exposed to the most extreme temperatures by day and night. 

 On the plains in India, on the top of the Himalayas, in Central Asia, in 

 Australia, in ftict everywhere where the air is dry, we find great heat 

 during the day, forming a dangerous contrast with the cold night fol- 

 lowing. In the dry air of the Desert of Sahara the change is so great 

 as to sometimes carry the temperature to freezing, or even to the for- 

 mation of ice. These known phenomena are in definite relation with 

 the small amount of moisture contained in the air of those regions.^ 



The more vapor there is in the air, the easier it is for a part of it to 

 condense in the form of rain, snow, fog, or dew. 



Aside from the climatological importance of moisture in the air, it is 

 not immaterial to plants whether the air is moist or dry, because dry- 

 ness increases the transpiration of water from the leaves, buds, stems, 

 &c., while moisture in the air retards this process, in degree correspond- 

 ing with its amount. 



In a moist atmosphere the soil is fresher and more productive than 

 in an arid one, because the evaporation of water in the soil takes place 

 more slowly, and it holds more from watery precipitation than the dry 

 air. 



'A reason for the apparently greater warmth of the -woodlands than the open fields, 

 as judged by our seusatiou, vrill be explained further on in speaking of the humidity 

 of the atmosphere, and the eftect of dry winds. 



^ The contrasts between the temperatures of day and night, on our western plains, 

 has been noticed by all travelers who have crossed them, and they have been often 

 described by writers ; yet from the dryness of the atmosphere, the transpiration from 

 the surface is more rapid, and this itself is a cooling process, so that a person would 

 scarcely /eeZ a temperature of 110^ more than one of 90^ in the humid climates of the 

 sea-coast. In these dry regions, colds are seldom tasen by exposure to the night air, 

 and in certain pulmonary diseases, a salutary effect is observed, if relief is seasonably 

 sought. 



