FROM THE CHALK OF HERTFORDSHIRE. 
137 
6. Does the water-level vary at different seasons of the year, and 
how ? Has it diminished during the last ten years ? 
fe; 7. Is the ordinary water-level affected by local rains, and if so in 
how short a time ? And how does it stand with regard to the water 
in the neighbouring streams ? 
8. Analysis of the water, if any. Does the water possess any 
marked peculiarity ? 
9. Nature of the rock passed through, including covering of drift, 
with thicknesses. 
10. Does the cover of drift over the rock contain surface-springs ? 
11. If so, are they entirely kept out of the well? 
12. Are there any large faults known to exist close to the well? 
Dor the complete elucidation of the phenomena of underground 
water it is necessary to combine with the points investigated by 
the British Association Committee with regard to wells, results of 
observations of rainfall, evaporation, and percolation, for each of 
which a different kind of gauge is required. 
The rain-gauge should preferably be five inches in diameter,* 
though gauges eight inches in diameter are frequently used. At 
Bothamsted Messrs. Lawes and Gilbert have circular gauges five 
and eight inches in diameter, and a rectangular gauge 7£ feet by 
6 feet, being —ok)— of an acre in area. The top of the gauge should 
be one foot above the surface of the ground, and it is advisable to 
have a deep rim above the funnel in order to collect snow. The 
receiver should hold at least six inches of rain, but, for the collection 
of small quantities, there should be an inner collecting-can or 
bottle, which in very heavy falls of rain will overflow into the 
outer receiver. 
The evaporation-gauge should be at least six feet square and two 
feet deep, as the water in smaller gauges gets unduly heated and 
they register too much. As an area of i okro of an acre has been 
adopted for both rain-gauges and percolation-gauges, it would be 
well to adopt the same area for evaporation-gauges, which might 
be rectangular (say 7 feet 3 inches by 6 feet, or 6 feet 7£ inches 
square), or circular (7 feet inches in diameter); but the precise 
area is of no consequence, if sufficiently large. The water should 
be kept up to within three or four inches of the top, the gauge 
being sunk in the ground to about this height, so that the water is 
usually nearly level with the surface of the ground. The amount 
of evaporation is determined by the decrease in the height of the 
water when no rain has fallen, and when rain has fallen by adding 
the decrease in height to, or subtracting the increase in height from, 
the depth of rain. A rain-gauge is therefore a necessary adjunct 
to an evaporation-gauge. 
The percolation-gauge may be of any size from 18 inches in 
diameter and from three feet to six in depth, as at Nash Mills. 
At Lea Bridge Mr. Greaves used gauges three feet square, and at 
Bothamsted Messrs. Lawes and Gilbert are experimenting with 
* This is the size recommended by Mr. G. J. Symons, F.R. S. 
