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approach more nearly to an equality; and in summer, as 
the day advances, the excess of the river temperature is soon 
counteracted by the rapid rise of that of the air, causing the fog 
of the early morn to be speedily taken up and dissipated unless 
the air is approaching to saturation from other causes, when rain 
more or less heavy usually ensues. 
If we next look to the mean number of days in each month in 
which the dry-and wet-bulb thermometers are coincident, or 
within one degree of being so, at the hour of 9 a.m. when the 
observations are made, we find that number highest in the 
autumn and winter months, the same seasons in which the fogs 
preponderate. And these two things usually occur together at 
such periods of the year; that is to say, if the dry-and wet-bulbs 
coincide there is generally fog and vice versa. In point of fact, 
however, the fogs are much more numerous than the coincidences 
of dry-and wet-bulbs at the above hour would indicate, the tempera- 
ture of the air having often by that time risen two or three 
degrees above the wet-bulb, while the fog is still thick, and such 
as to prevail a great part of the day. 
The coincidences of dry-and wet-bulbs in summer, arising more 
from humid westerly and south-westerly winds than from noctur- 
nal cold, are almost always either attendant upon rain falling at 
the time, or followed by rain in the course of the day—sometimes 
by thunderstorms. This brings us, however, to- the subject of 
the rain-fall which calls for a separate consideration. 
 Rain.—Of all the elements of climate none is more variable 
than the rain-fall, the amount differing exceedingly at different 
places, even confining the remark to our own island. The 
western counties have a much larger fall than the eastern, from 
the circumstance of our chief rains coming to us with a south- 
westerly wind, which in its passage over the Atlantic often 
becomes saturated or nearly saturated with moisture: the rain- 
clouds thus formed discharge their contents on the western shores 
