HAS OUR CLIMATE CHANGED? 66 7 



this is no more than might have been expected from a correct con- 

 sideration of the meteorological circumstances under which rain is 

 produced. 



It is the belief of European meteorologists that the mean rainfall 

 on the western portions of that continent varies little, if at all, when 

 periods of many years are considered. In England there are rainfall 

 records reaching back to 1677. Since 1725 these records are unbroken ; 

 at present there are more than 1,500 rain-gauge stations in that coun- 

 try. The Scotch observations extend to 1731, the Irish to 1791. 



A discussion of the observations made at the Royal Observatory 

 at Greenwich, in 1859, led to the conclusion that the annual fall of rain, 

 as compared with that previous to 1815, was becoming smaller; but 

 more extended observations, taken from gauges at stations widely 

 separated, led to the opposite conclusion, viz., that there was a perfect 

 compensation, the decrease at one place being compensated by the 

 increase at another. 



This conclusion was strikingly illustrated by the Continental ob- 

 servatories. The rainfall at Paris was found not to have altered in 

 130 years, and, though the observations of 50 years at Marseilles gave 

 a decrease, those for 54 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 

 1S59, was found by Mr. Symons to be five per cent, less than during 

 the previous 40 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 agricultural surface changes have been occurring 

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

 us how insignificant is the meteorological result which these varia- 

 tions produce. 



The Available Supply of Water. 



The actual supply of water does not depend on rainfall alone. It 

 is diminished by evaporation and also by percolation. When the 

 condition of the atmosphere is such that, either by reason of the heat, 

 the prevalence of dry winds, and other such causes, the water that has 

 fallen is exposed to rapid vaporization, the available supply necessarily 

 becomes less. 



As regards percolation, much depends on the rate at which the rain 

 falls, and the contemporaneous condition of the surface of the ground. 

 The supply may come so rapidly that there is not time for it to soak 

 into the earth. In this manner the quantity that properly belongs to 

 a whole month may fall in the course of a few hours, and, rushing over 



