On the Rain arid Drainage - Waters at Rothamsted. 
323 
rentlj removed from the soil about 9 inches more water than 
had evaporated from the adjoining bare fallow ; the conditions 
of the experiment were, however, not all that could be desired. 
The powerful action of a crop in evaporating water from 
a soil is mainly due to the rapid transpiration of water through 
the leaves, which takes place in a growing plant under the 
influence of light ; the roots also lend important assistance by 
enabling the plant to draw water from depths of the soil too 
gi-eat to be disturbed by ordinary capillary attraction. A deeply 
rooted crop may thus be more effective in drying the soil than 
a crop with shallow roots, as is plainly seen by a comparison 
of the results produced by the grass and barley-crops already 
mentioned. 
As the transpiration of water in a plant is determined by 
light, the amount of transpiration must have some connection 
with the rate of assimilation and growth. When the supply of 
water and of soluble plant-food is tolerably constant, the rela- 
tion between transpiration and growth will be fairly regular. 
From experiments made at Rothamsted many years ago, with 
plants grown in pots, it was concluded that from 250 to 300 lbs. 
of water were evaporated for 1 lb. of dry matter added to the 
plant. It may be, however, that in a soil poor in soluble plant- 
food a larger amount of water would pass through the crop to 
yield the same amount of assimilation than in the case of a soil 
well manured. The relation between transpiration and assimila- 
tion will, indeed, probably differ under different circumstances. 
The annual evaporation from a cropped soil can never be 
reckoned as a constant quantity, even under a uniform course of 
cropping, as the character of the season will greatly affect the 
growth of the crop, and consequently its evaporating power. 
The evaporating power of a crop is also so often above the 
average rainfall of the period of its active growth, that it is only 
occasionally that the full extent of this power is manifested. 
We must now conclude this section with a word regarding 
the results of others in this branch of inquiry. 
Dr. Dalton, as far back as 1796, constructed a percolation- 
gauge, consisting of a cylinder 3 feet deep, filled with soil, and 
sunk in the ground to the level of its upper edge, arrangements 
being made for collecting and measuring the water which passed 
through. This mode of experimenting has been adopted by 
many observers, as M. Maurice, M. Gasparin, Messrs. Dickin- 
son and Evans, Mr. Greaves, Prof. Ebermayer,* Dr. Sturte- 
* A brief notice of the results of Maurice, Gasparin, Ebermayer, and others, 
with a summary of those obtained at Rothamsted up to that date, will be found 
in the Minutes of Proceedings of the Institution of Civil Engineers, Session 
1875-6, vol. SLY. part iii. 
