232 Transactions of the 



ence over wooded and un wooded districts, and the rainfall is not now 

 more or less than it ever was. 



The same conclusions may be drawn from the results of the temperature 

 records, the fluctuations of which are found to be quite uniform through- 

 out the entire country. Two or three years near eighteen hundred 

 and twelve are historically known as cold years; and a reference to 

 this period shows them to have been more extreme than any since, if we 

 except the present Winter of eighteen hundred and seventy-four-seventy- 

 five, in the Eastern States. Next in severity come the cold years eigh- 

 teen hundred and thirty- five— thirty-six, and eighteen hundred and 

 thirty-six-thirty-seven. The next coldest groups were eighteen hun- 

 dred and twenty-three-twenty-four, and eighteen hundred and forty- 

 three-forty-four. The high temperature groups are eighteen hundred 

 and twenty-five-thirty, eighteen hundred and forty-four-forty-eight, and 

 eighteen hundred and fifty-three. 



It would seem that there are two classes of non-periodic changes — 

 one less frequent and affecting longer periods, and another causing 

 changes above or below the general line of these long periods, and 

 belonging to periods of a year or two. Further than this we find no 

 results worthy of special mention from these long continuous observa- 

 tions over our vast territory, and hence infer that man's agency in 

 influencing either the temperature or aqueous precipitation is, as far as 

 we are able to judge, altogether insignificant. Similar inferences are 

 deducible from the results of the observations made on the Pacific slope. 

 In the comparatively brief records herewith presented, there is no evi- 

 dence discernible of progressive or retrogressive movements, either in 

 the temperature or in the rainfall. But the question of the rainfall, or 

 of the temperature, does not settle the question of humidity. The 

 humidity of the atmosphere depends not so much upon the amount of 

 precipitation in rain as upon the rapidity of the process of evaporation 

 and drainage; and it is here that man's agency proves instrumental in 

 modifying climate. Our country, which was once largely covered with 

 an unbroken forest, is now, to a great extent, denuded, the decrease of the 

 forests being at the ratio of seven millions of acres annually. The 

 rain, which was gradually conveyed by the leaves of trees to a dense 

 undergrowth and layer of fallen leaves and vegetable mould, absorbed 

 it like a sponge, and whence it was transferred by the roots to the 

 depths of the soil, now runs off by the nearest watercourses, leaving 

 no supply of water during dry weather. The restraint of evaporation 

 by the dense shield afforded by forest shade being thus removed, the 

 sun pours down upon the unprotected soil and rapidly evaporates the 

 superficial water. The natural consequence of all this is an increased 

 dryness of the atmosphere. This conclusion, which is arrived at from 

 general observation and practical knowledge, needs not the proofs that 

 physical science affords by means of the wet and dry bulb thermometers. 

 The facts are patent and intelligible to all, and can be measured in an 

 uncovered district by the sensible diminution of a mountain stream 

 after a day of intense sunshine. In California, on the eastern side of 

 our great valley, in places where the upper lands have been cleared of 

 trees, the rainwater descends impetuously in a torrent, leaving tiny 

 streams, which flow steadily for many days, so long as the sky remains 

 overcast, but cease altogether after a single day of sunshine. 



In this connection, I would add that the rains are not now either 

 lighter or heavier, or more fitful, than in former times, but there are fewer 



