AUTUMNAL HUMIDITY. 
39 
require a continual drought, they have to bear rapid changes of 
drought and moisture, and it is the same when they would require 
to be habitually moist. It is not the rain which falls in showers, 
whether those showers are of longer or shorter continuation, which 
is so hostile to the proper growth, and through that to the superior 
blooming of plants ; for, in those places of the world where the 
year is most seasonal, and the periods of drought are the longest 
and most intense, more rain generally, indeed one may say, in¬ 
variably, falls in the course of the year, than in the most rainy 
districts of Britain. Not only this, but the showers are heavier, 
falling in absolute torrents ; and yet the air is not so much satu¬ 
rated with moisture, even in the intervals of the heavy showers, 
as it is in Britain during the drought. But, instead of hurting 
the vegetation, by thus suddenly arousing it from a state of perfect 
repose, the growth which this stimulates is more vigorous, and the 
flowering more perfect. So much is this the case, that the change, 
from a plantless desert to a paradise of the choicest flowers, is 
made in a very short period of time. Nor is it only in tropical 
countries where the average heat is greater than in Britain, that 
this takes place ; for it occurs in places of North America, where, 
though the summer temperature is as high as that of the warmest 
parts of the tropical countries, the thermometer, in winter, falls 
much lower than the freezing point of water ; indeed almost as 
low as that at which mercury becomes a solid metal, and would 
bear hammering on an anvil. The change is also very rapid ; 
and in Russia and other parts of the north of .Europe, where, 
though the summer is comparatively hot, and the winter very 
cold, the average heat of the year is lower than that of the British 
islands. 
In the one of these examples, the mean temperature is higher 
than in Britain, and in the other it is lower 5 so that the rapid 
and sudden growth of vegetation cannot be owing to high tem¬ 
perature ; or the slow growth, and periods of unhealthiness, cannot 
be owing to low temperature. Dry air is the primary cause, or, 
at all events, the grand modifying circumstance in the one case, 
and humid air is the same in the other. We have already said 
that showers, however heavy and long continued, do not produce 
the deleterious effect, but that it is owing to continued moistness 
of the plants, and the infallible result, continual evaporation from 
their surfaces. But in the countries of heavy showers, alternating 
