OVER A WIDE EXPANSE, 103 



feet compensation, the decrease at one place being compensated by 

 the increase at another. 



"This conclusion was strikingly illustrated by the Continental 

 observatories. The rainfall at Paris was found not to have altered 

 in 130 years, and though the observations of fifty years at Marseilles 

 gave a decrease, those for fifty-four years at Milan gave an increase. 



" Even in the same locality this principle of compensation may be 

 noticed. Thus the rainfall in England, in the ten years from 1850 

 to 1859, was found by Mr Symons to be five per cent, less than 

 during the previous forty years, but during the following six years it 

 was five per cent, above the mean of the preceding ten. 



" It may, however, be supposed that conclusions which apply to 

 the old-settled countries of Europe, in which but few important topo- 

 graphical changes, through agricultural or other operations, have 

 taken place for many years, will scarcely apply to America, wherein 

 the clearing of land and agriculture surface-changes have been occur- 

 ring on a very extensive scale. The foregoing conclusions, however, 

 show us how insignificant is the meteorological result which these 

 variations produce." 



The observations are cited as evidence that there are cases in 

 which the extensive destruction of forests does not appear to have 

 perceptibly affected the quantity of the rainfall over a wide expanse 

 of country ; and these observations are brought forward as 

 observations not less necessary to be taken into account, in 

 considering the effects of forests on the humidity of the climate, 

 than are those which have previously engaged our attention. 



SacT. II. — Cases in which the Extensive Destruction of Forests 



appear to have heenfolloioedhy a marked Desiccation 



of Land and Aridity of Climate. 



Lawyers have been credited with the hon mot that the case being 

 altered that alters the case, but the principle holds true in science 

 as well as in law ; and it seems to be a truth of universal application. 

 Along with the recorded observations which have been cited as cases 

 in which the extensive destruction of forests does not appear to have 

 perceptibly aflFected the quantity of rain falling over a widely- 

 extended country, there are other cases in which the extensive 

 destruction of forests appears to have been followed by a marked 

 desiccation of land and aridity of climate ; and these are so 



