22 METEOROLOGICAL OBSERVATIONS 



contains on the average 2*57 per million of sulphuric acid 

 (as SO3), and that the total annual amount per acre is 

 17*41 lb. The most noteworthy result is the close agreement 

 between the amounts furnished by the summer and winter 

 rain, especially in view of the great variations in the chlorine. 



In conclusion, it may be pointed out that the rain falling 

 at Rothamsted contributes to the soil enough chlorine and 

 sulphimc acid to meet the requirements of most crops. 



Proportion of Rainfall 'percolating through Bare Soil. 



Alongside the large rain gauge, three percolation or drain 

 gauges were constructed in 1870. Portions of the undisturbed 

 soil, each one-thousandth of an acre in area, were isolated 

 from the surrounding soil by digging trenches and building 

 brick and cement walls round the blocks of soil thus exposed. 

 The blocks were then undermined, and eventually carried 

 upon bars and plates of iron perforated to enable the percolat- 

 ing water to find its way into the collecting funnel beneath. 

 Thus in the end three blocks of undisturbed soil were 

 obtained, each one-thousandth acre in area, 20, 40, and 60 

 inches in thickness respectively, entirely isolated from the 

 surrounding soil, and the rain-water percolating through each 

 block is collected separately and measured like the rainfall. 



Table X. shows the average results obtained during the 

 thirty-four years 1871-1904. 



It will be seen that the three different thicknesses of soil 

 yield practically the same results, it being difficult to account for 

 the small but constant differences which occur. On the average, 

 about half the annual rainfall percolates through the gauges, 

 and about one-half is evaporated. It should be borne in mind, 

 however, that the surface of the soil in these gauges is kept 

 free from vegetation of all kinds, so that there is no drying 

 effect due to the crop. Again, as communication between 

 the subsoil and the soil of the gauges is cut off, all capillary 

 movements of water, both downwards during rain and back 

 again during periods of drought, are stopped at a certain point. 



