HfFLTJEKCE ON PEECIPITATIOIT. 193 



The effect of the forest on precipitation, then, is bj no means 

 free from doubt, and we can not positively affirm that the total 

 annual quantity of rain is even locally dhninished or increased by 

 the destruction of the woods, though both theoretical considera- 

 tions and the balance of testimony strongly favor the opinion that 

 more rain falls in wooded than in open countries. One import- 

 ant conclusion, at least, upon the meteorological influence of for- 

 ests is certain and undisputed: the proposition, namely, that, 

 within their own limits, and near their o^\ti borders, they main- 

 tain 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 precipitation, they probably equal- 

 ize its distribution through the different 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 rainfall in the centre 

 of the wooded district of Ciuq-Tranchees, 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. Fautrat and Sartiaut 

 placed at an elevation of six metres above a grove of oaks and elms, of twenty 

 years' growth and eight or nine metres height, in the forest domain of Hallete, 

 pluviometers and other meteorological instruments, and like instruments at 

 the same elevation in the open ground three hundred metres from the forest. 

 In the months February to July inclusive, the rainfall above the trees was 

 found to be 192.50'"™, and during the same period IT?'""" in the open ground. 

 —Comptes Rendus, t. LXXIX., 409. 



The alleged augmentation of rainfall 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 commonplace 

 in all discussions of the question. It is, however, open to the same objection 

 as the alleged instances of the diminution 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 

 9 



