and its Discharge by Drains. 
149 
per cent, of the rain which falls from October to March inclusive, 
passes back to the atmosphere by evaporation in the same period ; 
whereas from April to September inclusive, about 93 per cent, is 
evaporated. It appears, then, that there is even a balance on the 
side of rain over evaporation during the six hottest months ; and 
we discover only two years, J 840 and 1841, in which no filtration 
occurred within that period. Table II. shows that in August the 
soil is in its driest state ; but, even in that month, some filtration 
took place in 3 out of the 8 seasons recorded. It will be under- 
stood, that though a near balance is shown to subsist between rain 
and evaporation during the six hottest months, on an average of 
years, the hygrometric condition of a soil, i. e. its state of wetness 
or dryness at any particular time, is not indicated by the Dalton 
gauge. A soil may be in a state of drought, or of humid satura- 
tion at different times during these months, and according to the 
season. It is, however, manifest, from these registers, that if all 
the water derived from rain during the six colder months were 
allowed to accumulate in a soil, such land must be perpetually 
wet ; and coupling this fact with the performance of drains, which 
I am now enabled to exhibit, it appears that six months are ex- 
pended in maintaining, by the sole unaided force of evaporation, 
an undrained retentive soil in a tolerably uniform moist condition, 
whilst deep covered drains relieve the same soils of excess of 
humidity in a very few hours after every fall of rain, even in the 
wettest season. Table IV. shows that the mean excess of rain- 
water to be disposed of during the six coldest months by some 
other process than evaporation, amounts to no less a weight than 
about 1050 tons per acre. 
Evaporation is the only natural agent for diminishing the quan- 
tity of water absorbed by retentive soils, but it is not at our 
command. When such soils are perfectly saturated, the super- 
fluity must either stagnate on the surface or flow away from it ; 
and proof is here offered that the force of evaporation is scarcely 
equivalent to the duty required of it during one half of the year ; 
also that it greatly falls short of the requisite power during the six 
colder months. The invention of subterranean drains supplies an 
effective artificial method of compensating the deficiency of the 
evaporative force in our climate, and it is capable of placing the 
retentive soil in the same favourable condition, as respects meteor- 
ological agency and the fruition of every agricultural process, as 
soils naturally endowed with sufficient porosity. But, it must con- 
stantly be borne in mind, that, in order to assimilate this artificial 
process to that of nature, drains should be deeply laid, as the floor 
of the drains forms the limit of their action, and determines the 
depth below the surface at which water must still remain in a 
stale of nearly constant excess and stagnancy. 
