CHAPTER XXIV. 171 



TWO-FOLD EFFECTS OF FORESTS. 



The problem before us is to trace what influence forests may ex- 

 ert upon these points ot rainfall, evaporation and floods, and see what 

 foundation there is for the almost universal belief that they increase 

 the one and decrease the other two. 



Many meteorological stations in connection with forestry have 

 been established in France, in Germany, in India, and elsewhere. 

 In this country the subject has also received attention, but on a 

 more limited scale. Progress in the conclusive establishment of 

 definite effects has necessarily been slow. It is an unfortunate 

 fact in regard to all subjects connected with rainfall, that sys- 

 tematic observations must have been conducted over a long series 

 of years, forty to sixty, in order to obtain data upon which to predi- 

 cate positive results. The records of a few years may be, and they 

 frequently are, very misleading. The secular meteorological changes 

 tend to move in cycles of wet periods and drouth. No conclu- 

 sions can be safely accepted that are not based on records extending 

 sufficiently back into the past to include and give full weight to these 

 cycles. 



RAINFALL IN PHILADELPHIA. 



The records of precipitation at Philadelphia extend back to 1825. 

 Charting these precipitations gives a wave-like curve descending very 

 low in 1825, but with its sinuosities all well above that point until 1881, 

 when the minimum precipitation of 29.57 inches in 1825 was again 

 closely approached. The 1881 fall was 30.21 inches. The years of 

 maximum precipitation were 1841 and 1867. The cycle of extreme 

 low precipitation had a longer period than that of extreme high — 56 

 years against 26 years. 



Taking a total period of 64 years, and averaging the annual rainfall 

 by periods of four, eight, sixteen and thirty-two years, the averages 

 by four years run from 22 per cent, low to 19 per cent, high, as com- 

 pared with the entire period of 64 years. The eight-year grouping 

 gave results from 11 per cent, low to 11 per cent, high; the sixteen- 

 year groups, 6 per cent, low to 9 per cent, high, and the thirty-two-year 

 2 per cent, low to 2 per cent. high. 



LOS ANGELES RAINFALL. 



Analyzing the recorded rainfall at Los Angeles for the past twenty- 

 seven years, and averaging by periods of five years, gives results rang- 



