and its Discharge by Drains. 
153 
WCUpied in the discharge of the water by the gauge and the 
drains may, therefore, be considered to be identical, and as com- 
prising about 48 hours from the commencement of the rain. In 
drawing this parallel between the action of the gauge and these 
drains, I am presuming that the fall of rain at Penshurst was 
equal to that at King's Langley ; and I think this may be assumed 
to be near enough to the truth, as I have learnt that a nearly 
similar downfall (I'^ths of an inch) was recorded at Birmingham 
northwards, and a rain of similar duration occurred at Brighton 
southwards. 
This experimental corroboration of the sufficiency of such small 
drains wdl have its weight with practical men ; but I am further 
able to demonstrate, by simple arithmetical computation, how very 
small is the quantity of water required to enter the crevice formed 
by the imperfect junction of two pipes. The rain-gauge informs 
us, that yVoths of an inch in depth of rain fell upon each square 
foot of surface^ in the observed time of 12 hours. This quantity 
is equivalent to 69tV cubic inches, or 2^ pounds, which, divided 
by 12 hours, gives little more than i^^ths of a pound per square 
foot of surface per hour for the weight of the rain. 
The drains w ere 24 feet asunder, and each pipe a foot in length, 
so that each lineal foot had to receive the water falling on 24 
square feet of surface, equal to 60 pounds, or 6 gallons ; and as 
the time which this quantity occupied in descending through the 
soil and disappearing was about 48 hours, it results that 1| lb., 
or one pint per hour, entered the drain through the crevice exist- 
ing between each pair of pipes. Every one knows, without having 
recourse to strict experiment, how very small a hole will let a pint 
of water pass through it in an hour, being only one -third of an 
ounce per minute, or about twice the contents of a lady's thimble. 
The weight of rain, per acre, which fell during the 12 hours 
amounted to 108,900 lbs., or 48tV tons, which on the whole piece 
of nine acres, is equal to 437tV tons ; and each drain discharged 
19 tons, equal to about ^^ths of a ton per hour, on the mean of 
48 hours ; but when the flow was at the greatest, I find that each 
drain must have discharged at the rate of five times this quantity 
per hour, which affords proof of the faculty of the pipes to receive 
and carry off a fall of rain equal to 2| inches in 12 hours, instead 
of half an inch, a fall which is quite unknown in this climate. 
Half an inch of rain in 12 hours is a very heavy rain. I learn 
from Mr. Dickinson, that his rain-gauge has never indicated so 
great a fall as 1^ inch in 24 hours ; and from Dr. Ick, the cura- 
tor of the Birmingham Philosophical Institution, that only on five 
occasions has the rain there exceeded 1 inch in 24 hours, during 
the same period of 8 years ; the greatest quantity having been 
1-1*^ inch on Dec. 4, 1841. We may, therefore, consider the fact 
