CLIMATIC CHANGE. 
21 
husbandry, or tlie diversion or choking up of natural water¬ 
courses, it may become more highly charged with humidity. 
An increase or diminution of the moisture of a soil almost 
necessarily supposes an elevation or a depression of its winter 
or its summer heat, and of its extreme, if not of its mean 
annual temperature, though such elevation or depression may 
be so slight as not sensibly to raise or lower the mercury in a 
thermometer exposed to the open air. Any of these causes, 
more or less humidity, or more or less warmth of soil, would 
affect the growth both of wild and of cultivated vegetation, 
and consequently, without any appreciable change in atmo¬ 
spheric temperature, precipitation, or evaporation, plants of a 
particular species might cease to be advantageously cultivated 
where they had once been easily reared.* 
some cases, these tracts compose basins apparently scarcely at all exposed 
to any local influence ifl the way of percolation or infiltration of water 
toward or from neighboring valleys. But in such situations, apart from 
accidental disturbances, the ground is growing drier and drier, from year 
to year, springs are still disappearing, and rivulets still diminishing in their 
summer supply of water. A probable explanation of this is to be found 
in the rapid drainage of the surface of cleared ground, which prevents the 
subterranean natural reservoirs, whether cavities or merely strata of bib¬ 
ulous earth, from filling up. How long this process is to last before an 
equilibrium is reached, none can say. It may be, for years ; it may be, for 
centuries. 
Livingstone states facts which favor the supposition that a secular 
desiccation is still going on in central Africa. When the regions where 
the earth is growing drier were cleared of wood, or, indeed, whether 
forests ever grew there, we are unable to say, but the change appears to 
have been long in progress. There is reason to suspect a similar revolution 
in Arabia Petrosa. In many of the wadis, and particularly in the gorges 
between Wadi Feiran and Wadi Esh Sheikh, there are water-worn banks 
showing that, at no very remote period, the winter floods must have risen 
fifty feet in channels where the growth of acacias and tamarisks and the 
testimony of the Arabs concur to prove that they have not risen six feet 
within the memory or tradition of the present inhabitants. Theie is little 
probability that any considerable part of the Sinaitic peninsula has been 
wooded since its first occupation by man, and we must seek the cause of 
its increasing dryness elsewhere than in the removal of the xoiest. 
* The soil of newly subdued countries is generally in a high degree 
