32 
H. G. FOKDHAH-WATER-LEVEL IN' 
from the area draining towards the south into the Thames, and 
upon one of the highest points of that ridge. The surface-flow of 
water at Therfield Rectory is actually towards the south, while the 
probable flow of the underground water is to the H.jST.W., to the 
springs in the valley of the Cam. 
The nearest of these springs to the three wells respectively, upon 
a line drawn from each between H. and N.H.YY., is, from Odsey, a 
spring to the E. of Ash well, distant If miles; from Therfield, one 
near Litlington (Cambs), distant nearly 4 miles ; and from Barley, 
a spring at Melbourn (Cambs), also about 4 miles from the well. 
The level of the springs above the sea would be approximately 150 
feet: the wells are sunk from a level of 506 feet at Therfield, 305 
at Barley, and 265 at Odsey, and their depths are 276, 165, and 
104 feet respectively. 
With regard to their position geologically, a section given in 
Hr. Elsden’s paper ‘ On the Post-Tertiary Deposits of Herts ’ * may 
be used to assist my explanation. This section is drawn N. and S. 
across the Cam Valley and the Chalk outcrop. If we consolidate 
three parallel sections drawn N. and S. through the three wells into 
one, and place the Therfield well at “ Heed Green,” to which it is 
actually very near, Barley will be lower on the outcrop about equi¬ 
distant between “Reed Green” and “Royston,” and Odsey at about 
the level of “Royston.” The line of springs will then be to the 1ST. 
of “Royston” again. It will be seen at once that the general level 
of water in the wells on the slopes of the Chalk hills to the south¬ 
ward depends upon the level of this line of springs, a level which 
is practically uniform and stationary, varying only, at most, to the 
extent of a few feet between the wettest and the driest seasons. 
In the Chalk hills to the south of this line of quasi-overflow the vari¬ 
ation of level in the undergroundwater becomes greater, the records 
giving a maximum difference between highest and lowest level of 
44^ feet at Odsey, 78 at Barley, and 64f at Therfield; the greatest 
difference in any one year (April to March) being respectively 
391 feet (1882-3), 52 feet (1865-6), and 34 feet (1884-5). 
The daily record at Odsey is necessarily too voluminous for pub¬ 
lication in detail. I therefore content myself with a table of the 
levels on the first of each month, arranged with the commencement 
of the year on April 1st, the date at or near which the water-level 
has attained its maximum elevation as the result of the autumn and 
winter rainfall, and from which, under normal circumstances, a 
uniform fall in level continues throughout the summer and autumn.f 
To this table is added the mean level for each month for the 
series of years, and the mean level for each individual year. 
Eor each water-level year is given also the rainfall for the year 
ending December 31st, so that the rainfall is for a period of twelve 
months, terminating three months earlier than that taken for the 
level of the water in the well. 
* ‘ Transactions,’ Vol. I, p. 105 . 
t See the chart of mean monthly level at Barley and Odsey, ‘ Transactions,’ 
Yol. Y, p. 22. 
