Experimental. Crops at Rothanisted. 131 
the water ; and there was an arrangement of tubes for the escape, 
and measurement, of the drainage water. Grass was grown on - 
the surface of the soil in the cylinder. The drainage woukl 
doubtless be more free in the experiments of Mr. Dickinson than 
in those of Dr. Dalton ; and the results, over 8 years, showed, with 
a less rainfall, a larger actual amount of drainage ; the latter 
representing 42J per cent., and the evaporation, therefore, only 
57i per (cnt. of the rainfall. This amount included, of course, 
the exhalation due to vegetable growth. 
From results obtained by gauging the flow of water from pipe- 
drains, it has been concluded that a still larger proportion of 
the rainfall passes off by evaporation than that indicated by the 
experiments of either Mr. Dickinson or Dr. Dalton. But results 
obtained by deducting the amount passing through drains from the 
total rainfall may be judged to be quite untrustworthy', from 
the fact that, before the pipe-drains in the experimental wheat field 
had passed any water at all in the autumn of last year, the drain- 
qauqcs already referred to had indicated that, of the rain which 
had then fallen since the 1st of September, nearly 25 per cent, 
had passed below 20 inches, nearly 10 per cent, below 40 inches, 
and nearly 4 per cent, below 60 inches from the surface. It is 
clear, therefore, that the amount of water passing through arti- 
ficial drains may be no measure whatever of the total quantity 
passing below the reach of the roots of growing crops. 
In the admitted defect of satisfactory evidence from which 
may be deduced the probable average amount of evaporation 
from the surface of the soil independently of vegetation, we will 
assume, by way of illustration, that, taking the average of many 
soils and seasons, three-fourths of a total rainfall of 25 inches 
will pass off by the combined action of evaporation from the sur- 
face of the soil itself, and of the exhalation due to the growth of 
a good crop of hay or corn. On this supposition there would 
still remain more than 6 inches of rain, equivalent to more than 
600 tons of water per acre, annually passing downwards, and 
carrying with it more or less of fertilising matters. 
Fortunately, some of the most important mineral constituents 
of soils and manures are, in the case of the heavier soils at any 
rate, almost wholly retained by them within the range of the roots 
of our crops. Nitrogen, whether supplied in the form of ammonia- 
salts or nitrates, is, however, much less completely so retained, 
being, in whichever state supplied, carried off in greater or less 
quantity in the drainage water, chiefly in the form of nitrates. 
According to results obtained independently by Professor Frank- 
land and Professor Voelcker, on the analysis of drainage water 
from the experimental wheat field at Rothamsted, that col- 
lected during the winter, from land manured in the autumn by 
an amount of ammonia-salts supplving 82 lbs. of nitrogen per 
2 
