234 
They are at a loss to understand it. But the dewpoint if attended 
to would often explain the circumstance. This would probably 
be found to have risen, speaking to an increased humidity of the 
air, in consequence of a westerly or south-westerly current 
suddenly mixing with a colder stratum in some part of the 
atmosphere, and causing condensation. The introduction of such 
a current would not necessarily entail a fall of the barometer, as 
northerly currents at higher elevations might still so prevail as to 
keep up the pressure. The rain in such a case, however, would 
probably be only for a few hours. 
At another time the barometer may be observed falling for a 
whole week without any rain occurring till near the close of it, 
the wind and temperature remaining all the while unaltered, 
Such a circumstance often attends the breaking up of fine weather 
after a dry period of long continuance. It points to the gradual 
substitution of a south-westerly wind for a north-easterly, the 
moister and warmer current setting in first in the higher strata of 
the cloud region, the strata next beneath slowly revolving in 
succession after it, the stratum just above the earth not being 
affected till the last, when the vane suddenly turns round, the 
temperature rises, and rain ensues. The gradual increase of 
humidity in the air, as the south-westerly current works its way 
more and more into the lower strata, would here be revealed by 
the wet-and dry-bulb thermometer its movements synchronizing 
to some extent with the descent of the barometer, though no 
marked change of weather might follow for several days. 
As considerations on the vaporous portion of our atmosphere 
serve to explain states of weather which would not be otherwise 
intelligible, so also do they assist us in forecasting the weather 
that is to be during the day. Whether it shall be wet or fine 
often depends upon the temperature and dewpoint in their 
relations to each other,—upon the fall or rise of one of the two, 
the other perhaps remaining unchanged,—though sometimes, no 
doubt, the result, whichever it may be, is due to influences beyond 
