324 On the Rain and Drainage- Waters at Rothamsted. 
vant, &c. It is obvious that on this plan the soil forming- the 
drain-gauge is more loose and open in texture than the natural 
consolidated soil of a field, thus admitting a freer percolation ; 
pains also have seldom been taken to include the natural sub- 
soil in the percolation-cylinder, which has generally been filled 
entirely with a surface soil. The surface of the gauge has 
again not been interfered with, and has speedily become covered 
by a mass of grass and weeds. The evaporating power of the 
soil is of course greatly increased by the presence of this vege- 
tation. Of the results obtained by the use of Dalton gauges 
by far the most extensive are those by Mr. Dickinson, of Nash 
Mills, Hemel Hempstead, Herts, commencing in 1836,* and 
latterly continued by Mr. J. Evans ; and those commenced by 
Mr. C. Greaves at Lee Bridge in 1851, and carried on to the 
present time. We shall refer to the results of these experi- 
menters in some little detail, as they excellently illustrate the 
influence of a crop on percolation and evaporation. 
Messrs. Dickinson and Evans have employed two drain- 
gauges, consisting of cast-iron cylinders 3 feet in depth, and 
18 inches in diameter; one is filled with the surface soil of the 
neighbourhood, the other with fragments of chalk ; both bear 
a growth of grass. Mr. Greaves's drain-gauges consist of two 
square boxes made of slate, 3 feet in depth and 3 feet square ; 
one of these is filled with sand (such as is employed for filter- 
beds, passing through a screen of 33 No. 10 wires in 6 inches) 
to within 2 inches of the top ; the other with a mixture of soft 
loam, gravel and sand, trodden in and turfed. Mr. Greaves has 
also a gauge for measuring the evaporation from a water-surface, 
consisting of a tank 1 loot in depth, and having an area of 
1 square yard ; this tank is kept afloat in a flowing stream. 
The tank contains a few inches of water, the rise or fall in 
which is ascertained from time to time. This is probably the 
most accurate method of determining the rate of evaporation 
from water yet adopted. The figures we shall quote are taken 
from two papers read by Mr. Greaves and Mr. Evans before the 
Institution of Civil Engineers, February 29, 1876 ; these supply 
us with the results of Mr. Greaves for fourteen years — 1860-73, 
and those of Mr. Evans for fifteen drainage-years — 1860-1 to 
1874-5. The following Table (p. 325) gives a summary of 
Mr. Greaves's results : — 
The mass of sand which fills one of the percolators supplies 
an extreme example of a soil of the lowest water-holding and 
capillary power ; the rain passes through it without hindrance. 
* The results for the first eight years will he found in the volume of this 
' Journal ' for 1845, page 150. 
