with Geographic Botany. — Humidity. 



321 



or even of the day. In the latter months of our summer a 

 good deal of rain falls, and this is greater as the previous 

 weather has been warm, and the air become saturated with 

 moisture ; the heat of the summer has favoured the absorption 

 of an unusual quantity of aqueous vapour, and ultimately the 

 air becomes so saturated that a small decrease of temperature 

 produces precipitation. 



The quantity of vapour dissolved in the atmosphere from 

 the equator to high latitudes is very regular in its progression, 

 and we are fortunate in being enabled to maintain this posi- 

 tion by a reference to an extensive series of observations, in the 

 Appendix to Beechey's ^ Voyage,^ from a small portion of which 

 the next table has been compiled. A period has been selected 

 when the continuity of the observations was very little broken, 

 and which embraced high latitudes in both hemispheres ; the 

 whole were obtained in the Pacific Ocean. 



These results are obtained from a number of daily observa- 

 tions, meaned to every 5° of latitude, from which the weight 

 of vapour has been calculated. The whole of the details offer 

 much material for comparison ; they include a period of four 



