29 
springs, and the yield of the springs in any one year 
depends upon the amount of water stored in this zone. 
In cases where the wells are shallow the middle zone 
is absent. When replenishment of the springs takes 
place, it is not the immediate rain that passes into the 
lower zone, but a given volume of rain percolating 
drives out an equal quantity of water held by capillarity 
in the earth, and so rain is held in the earth in some 
cases for many years before it flows out again as springs, 
but the flow in any one year is dependent upon the 
amount of rain previously percolating in that particular 
season. The mineral constituents of water that has 
been held so long in the ground vary but slightly from 
year to year regardless of the volume flowing. 
“¢ Water held in the ground contains air and gases 
and the rocks also contain them. This being so, a 
reduction in barometric ‘pressure has a tendency to 
cause this air or gas to escape from or expand in the 
water, or at other times for the water to retain under 
high barometric pressure a larger amount or to compress 
the air or gas present. The effect, therefore, of a fall 
of the barometer is to aid in the escape of water by 
reason of the expansion of the gases under low baro- 
metric pressure. On the other hand, under high 
barometric pressure the condensation of the gases leads 
to the retardation of the flow and the escape of the 
water. 
“In order to determine whether or not the actual 
flow of water from the ground was affected by baro- 
metric pressure, the Author commenced a series of 
observations upon the Bourne flow in the Caterham 
Valley in the present year (1881). For this purpose 
he had a gauge dam constructed across the stream, and 
the quantity of water flowing over was recorded by a 
Ground water 
and strata 
contain air, 
Observations 
on Croydon 
Bourne. 
