INFLUENCE OF PRECIPITATION. 197 
by the destruction of the woods, though both theoretical con- 
siderations and the balance of testimony strongly favor the 
opinion that more rain falls in wooded than in open countries. 
One important conclusion, at least, upon the meteorological 
influence of forests is certain and undisputed: the proposition, 
namely, that, within their own limits, and near their own bor- 
ders, they maintain a more uniform degree of humidity in the 
atmosphere than is observed in cleared grounds. Scarcely less 
can it be questioned that they tend to promote the frequency 
of showers, and, if they do not augment the amount of precipi- 
tation, they probably equalize its distribution through the dif 
ferent seasons.* 
* The strongest direct evidence which I am able to refer to in support of the 
proposition that the woods produce even a local augmentation of precipitation 
is furnished by the observations of Mathieu, sub-director of the Forest-School 
at Nancy. His pluviometrical measurements, continued for three years, 1866- 
1868, show that during that period the annual mean of rain-fall in the centre 
of the wooded district of Cing-Tranchées, at Belle Fontaine on the borders of 
the forest, and at Amance, in an open cultivated territory in the same vicinity, 
was respectively as the numbers 1,000, 957, and 853. 
The alleged augmentation of rain-fall in Lower Egypt, in consequence of 
large plantations by Mehemet Ali, is very frequently appealed to as a proof of 
this influence of the forest, and this case has become a regular common-place 
in all discussions of the question. It is, however, open to the same objection 
as the alleged instances of the diminuticn of precipitation in consequence of 
the felling of the forest. 
This supposed increase in the frequency and quantity of rain in Lower 
Egypt is, I think, an error, or at least not an established fact. I have heard 
it disputed on the spot by intelligent Franks, whose residence in that country 
began before the plantations of Mehemet Ali and Ibrahim Pacha, and I have 
been assured by them that meteorological observations, made at Alexandria 
about the beginning of this century, show an annual fall of rain as great as is 
usual at this day. The mere fact that it did not rain during the French 
occupation is not conclusive. Having experienced a gentle shower of nearly 
twenty-four hours’ duration in Upper Egypt, I inquired of the local governor 
in relation to the frequency of this phenomenon, and was told by him that not 
a drop of rain had fallen at that point for more than two years previous. 
The belief in the increase of rain in Dgypt rests almost entirely on the 
observations of Marshal Marmont, and the evidence collected by him in 1830, 
His conclusions have been disputed, if not confuted, by Jomard and others, 
