and its Discharge by Drains. 
157 
I may here observe, that Mr. Hammond, when draining tena- 
cious clays, chooses the month of February for the work, when 
he lays his pipes (just covering them with clay to prevent crumbs 
from getting in), and leaves the trenches open through March, 
if it be drying weather, by which means he finds the cracking of 
the soil much accelerated, and the complete action of the drains 
advanced a full season. The process of cracking may, doubtless, 
be hastened both by a choice of the period of the year in which 
drains are made, and by such a management of the surface as to 
expose it to the full force of atmospheric evaporation. 
Recurring to the foregoing Tables, it must be noticed that the 
mean annual fall of rain, as therein registered, is below the average 
of Britain, whilst the force of evaporation is probably higher than 
the average ; and the monthly as well as annual amounts of 
filtration and evaporation may be expected, in different latitudes, 
localities, and soils, to vary greatly from th«se records. Similar 
observations obtained on different soils, and in various parts of the 
country, when combined with the indications of thermometers sunk 
in the earth, would put us in possession of that condition of soil 
which may not be improperly termed its climate, of which no 
certain knowledge can be deduced from purely meteorological 
phenomena, but upon which the atmospheric climate of a district 
is known greatly to depend. 
Meteorologists have recorded, for many years, the amount of 
terrestrial evaporation, as denoted by a gauge invented by Mr. 
Luke Howard, and have considered it as " indicative of the quan- 
tity of moisture taken up by the atmosphere from the earth but 
this instrument only denotes the evaporation from a dish of water 
placed on the earth's surface, and, therefore, supplies no fact of 
direct use to the agriculturist, for cultivated soils are not under 
these circumstances, and the power of the sun's rays in heating 
soil is but indifferently represented by their effect in transforming 
water into vapour. The difference between the indications of 
the Howard and Dalton gauges is most remarkable. Professor 
Daniell states {British Almanac') the mean annual rain in Lon- 
don to be 22-199 inches, and the mean evaporation 23-981 inches, 
or 1-782 inches more than the rain ; and the results recorded at 
the Birmingham Philosophical Institution for 1843 are — rain 
26-716 inches, evaporation 31-982 inches, or 5 266 inches more 
than the rain. But we learn from the Dalton gauge that, in 
Hertfordshire, out of 26-614 inches of rain only 15-32 inches 
were restored to the atmosphere — the remainder passed through 
the earth into the rivers, and this is the real fact on comparing the 
amount of rain with the amount evaporated from soil 3 feet deep. 
We must never forget that accurate and miiltiplied quantitative 
facts form the only substantial basis of science ; and observations 
