On the Rain and Drainage - Waters at Rothamsted. 313 
months with a rainfall of 22"937 inches, and another similar 
period with a fall of 42"718 inches. In the six winter months 
the ranjje of rainfall has been still greater, namely from 7*031 to 
21*364 inches. The ten years of experiment have thus afforded 
examples of extreme rainfall and drought, such as are usually 
only found in much longer periods of observation. 
With this very large variation in the rainfall we have a yet 
greater variation in the amount of water passing through the 
soil. The summer drainage is seen to vary from 0 923 to 
12*271 inches; the winter drainage from 3*918 to 15*530 
inches ; and the drainage of the whole drainage-year from 
4*970 to 25*857 inches. Expressed in percentages of the rain- 
fall the drainage in summer has varied from 7 9 to 47*6, with 
a mean of 26*8 per cent. ; the drainage in winter from 39*8 to 
80*1, with a mean of 61*9 per cent. ; and the drainage of the 
whole year from 21*7 to 60*5, with a mean of 43*4 per cent. 
To understand the cause of this extreme variation in the 
amount of water passing through the soil we must turn our 
attention to the amount of evaporation from the surface which 
has at the same time taken place, and which is also shown in 
the table. The amounts of evaporation during each season have 
been ascertained by simply subtracting the amount of drainage 
from the amount of rainfall. That portion of the rainfall which 
does not appear as drainage-water has clearly been returned to 
the atmosphere by evaporation ; this is true, at all events, when 
the soil holds a similar amount of water at the beginning and 
end of the period of the experiment. That the soil of the drain- 
gauges was actually in a similar state of dryness at the beginning 
and end of every period mentioned in the table is by no means 
asserted ; the figures representing the evaporation are thus 
perhaps seldom quite exact, and in a few cases are certainly 
in error. The error is, however, usually small, as at the com- 
mencement and end of the periods chosen (October 1 and 
April 1) the soil will generally contain a moderate, but not 
excessive, amount of water. In the mean evaporations for 
summer and winter, given at the foot of the table, the error 
just mentioned will be but small ; and in the mean evaporation 
for the whole year probably nil. 
The amount of evaporation taking place from a bare uncropped 
soil will depend on the temperature of the soil, the temperature 
and dryness of the air, and the amount of wind, and also on the 
amount and distribution of the rain : the amount of evaporation 
is, in fact, limited not only by the conditions as to heat, wind, 
&c., but also by the amount of water available on the surface. 
The distribution of the rain has a considerable influence on 
the amount of evaporation ; a heavy rainfall occurring in a few 
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