RAINFALL AND MOISTURE. 77 



But it is not by rainfall alone that vegetation is pro- 

 moted. Account should be taken of the dew, and not of 

 the dew only, but of moisture in the atmosphere, which is 

 to the eye invisible, In a very ancient record anent cos- 

 mogony we read : * The Lord God had not caused it to 

 rain upon the earth, and there was not a man to till the 

 ground ; but there went up a mist from the earth, and 

 watered the whole face of the earth.' I cite this in illus- 

 tration of the fact that, for a long time indeed, it has been 

 a recognised fact that a mist or dew may promote vegeta- 

 tion. Dry as is a London fog, a Scottish mist is a connect- 

 ing link between this and the drizzling rain. I have 

 already had occasion to report the observations made on 

 the frequency of mists at different seasons in different 

 parts of Norway. 



The mist, the cloud, the dew, and the rain, are all pro- 

 duced by a fall of temperature, rendering the air incapable 

 of sustaining in its composition a quantity of moisture 

 which was previously there, but invisible ; and it may 

 sometimes be more important to determine the humidity 

 of the atmosphere than the rainfall, this being only a 

 deposit of surplus moisture in excess of what the air could 

 sustain in solution. 



While the pressure of the atmosphere is measured by its 

 equivalent in the weight of the column of mercury in the 

 barometer, the quantity of vapour in the air is measured 

 by its tension, which is also so determined. 



Taking the mean of the whole year, the greatest quan- 

 tity of aqueous vapour in the atmosphere of Norway is 

 found on the coast from the embouchure of the Christiania 

 fiord to the island of Karrnoe, where its tension is 6'5 mm. 

 A tension of 6* mm. extends over the middle of Christiania 

 fiord, over the Boemmel fiord to Bergen, at Stat, and on 

 the coast of Romsdal. The tension of 5 mm. passes a 

 little to the north of Christiania, over the Inner-Sogne to 

 the east of the Drontheim fiord, and away to the north, 

 following the coast of Nordland, up to theLofoden Islands. 

 The vapour tension in the interior of Southern Norway, 



