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 f 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. 



Date. 



Latitude. 



Tempe- 

 rature 

 of Air. 



Dew- 

 point. 



Weight of 

 Vapour in a 

 cubic foot of 

 Air. 



Winds. 



Sept. 25 to Sept. 29 



o 



55- 



-50 S. 



42°6 



36° 7 



grains. 

 2-9141 



Westerly 



Sept. 30 to Oct. 





50- 



-45 



45-8 



44- 



3-7365 



and 



Oct. 2 to Oct. 



3 



45- 



-40 



4948 



Precipitation. 



South-west. 



Oct. 4 to Oct. 



8 



40- 



-35 



53-31 



47- 



4-0767 





Nov. 1 to Nov. 



6 



35- 



-30 



62-59 



53-6 



5-0368 



Southerly. 



Nov. 7 to Nov. 



24 



30- 



-25 



69-8 



65-6 



7-2185 



Nov. 25 to Dec. 



5 



25- 



-20 



74-5 



71-9 



8-8272 



S.E. trade. 



Feb. 1 to April 25 



20- 



-15 



80-21 



77-9 



10-4883 





April 26 to May 



i 



15- 



-10 



81-34 



78-4 



10-3293 





May 2 to May 



5 



10- 



- 5 



82-60 



79-8 



9-6099 





May 6 to May 



8 



5- 



- 



80-9 



78-5 



10-9278 





May 9 to May 



11 



0- 



- 5N. 



80-24 



79- 



10-9945 





May 1 2 to May 



14 



5- 



-10 



78-85 



78- 



10-6745 





May 15 to May 



16 



10- 



-15 



78-97 



75-5 



9-8837 



N.E. trade. 



May 17 to May 



18 



15- 



-20 



78-2 



73- 



9-2363 





May 



19 



SandwichP. 



77-29 



70-5 



8-1921 





June 3 to June 



12 



25- 



-30 



75-49 



66-3 



9-3803 





June 13 to June 



18 



30- 



-35 



70-82 



67-7 



7-3920 





June 19 to June 



21 



35- 



-40 



73-27 



70-5 



8-5171 



Variable. 



June 22 to June 



24 



40- 



-45 



56-33 



59- 



6-6082 





June 25 to June 



27 



45- 



-50 



45-12 



45- 



3-7480 





July 6 to July 



11 



50- 



-55 



4710 



47-4 



4-1933 





July 12 to July 



15 



55- 



-60 



45-97 



46-5 



4-0713 





July 16 to July 



19 



60- 



-65 



43-52 



43-6 



3-6972 





Aug. 19 to Aug. 



26 



65- 



-70 



40-01 



38-7 



3-1634 



West. 



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 



