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 nota 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 Karmoe, 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 the Lofoden Islands, : 
The-vapour tension in the interior of Southern Norway;” 
