DRAINAGE AND CLIMATE 211 



The mean annual rainfall at the Greenwich Royal Ob- 

 servatory, according to the records 1815 to 1865 (55 

 years), 1 was 24.98 inches, and if the five-year period 1820 

 to 1824 is omitted, the mean rainfall was 24.4 inches. 

 The mean rainfall at Greenwich, 1825 to 1869 inclusive 

 (45 years), was 24.05 inches. It may be objected that 

 England is small in area and subjected, in large measure, 

 to ocean environment. 



A study of precipitation records reveals the fact that 

 mean annual rainfall varies in large cycles and that 

 apparently the mean of one cycle differs little from that 

 of another. The ground, therefore, for passing judgment 

 on diminishing or increasing rainfall is insufficient. 



283. Drainage and rainfall. The weather records of 

 Great Britain do not indicate a diminishing rainfall. 

 Meteorologists of this country do not admit an actually 

 diminishing rainfall in any part of the country, due to 

 drainage or any other cause. The only relation, there- 

 fore, that seems possible between drainage and rainfall 

 is that previously expressed, viz. : the conservation and 

 utilization of the precipitation that comes in the form 

 of rain or snow. 



284. Changing temperature. Students of climate 

 assert that there are no marked permanent changes occur- 

 ring in the mean temperature of any part of the world, 

 so far as records show. There may occur very marked 

 variations for a year or month. Taken in ten-year 

 periods for a series of years, the means for these periods 

 will not vary greatly from each other. The same may 

 be said of the temperature of any month by periods. The 

 following table includes the mean temperatures for the 



1 Dempsey and Clark, Drainage of Lands, etc., p. 101. 



