FROM THE CHALK OF HERTFORDSHIRE. 
141 
In questions relating to water-supply we have only to consider 
the average rainfall so far as it enables us to estimate the probable 
minimum, upon which we can alone rely; not necessarily in every 
case the minimum of a single year, but rather in most instances 
that of a short series of years, such as three; hence the value of 
these rules, the last especially. 
The mean rainfall at our oldest Hertfordshire station, Hash Mills, 
for the last half-century, has been 27*90 inches per annum. Taking 
each year as from the 1st of October to the 30th of September 
(a winter and a summer half), the driest year was 1863-64 with 
17- 66 inches; the driest two consecutive years were 1853-55 with 
an average of 20-55 inches; and the driest three consecutive years 
were 1862-65 with an average of 20-87 inches. Applying the 
rules of Mr. Symons, the figures should be within 7 per cent, of 
18- ^ ins., 20£ ins., and 22 ins., whereas they are really within 4 per 
cent, of these amounts, a remarkably close approximation. These 
rules strictly apply only to the rainfall at a single station, and in 
applying them to that of our county we shall, I think, be safe in 
ignoring the deviation allowed of 7 per cent., as an excess of that 
amount at some stations will probably be compensated for by a 
defect at other stations. With a mean annual rainfall of 25 inches 
we may therefore have a single year with only 16f inches of 
rain; two years in succession with an average of 18^- inches; and 
three years in succession with an average of 19f- inches. 
I am not aware that any observations on evaporation from water- 
surfaces that can be relied upon have been made in Hertfordshire. 
At Lea Bridge, only a short distance from the south-eastern 
boundary of our county, experiments were made by Mr. Charles 
Greaves during the 14 years 1860-73, the result from the 
records of a gauge 3 feet square and 1 foot deep, floating on a 
running stream, giving an evaporation of 20*61 inches per annum, 
with a rainfall during the same period of 25*72 inches, the rainfall 
thus exceeding the evaporation by 5*11 inches. At Strathfield 
Turgiss, in Hampshire, for the 14 years 1870-83, the Rev. 
C. H. Griffith observed the evaporation from a tank 6 feet square 
and 2 feet deep sunk in the ground, and determined the mean to be 
18*03 inches per annum, with a rainfall of 25*85 inches, the rainfall 
thus exceeding the evaporation by 7*82 inches. Mr. G. J. Symons 
has given, for the five years 1885-89, the results of observations of 
evaporation from the same tank in his garden in Camden Square, 
London, the tank having been removed there from Strathfield 
Turgiss, and he has found the mean to be 14*54 inches out of a 
rainfall of 24*89 inches, being a difference of 10*35 inches. These 
are the most satisfactory observations of evaporation from water- 
surfaces which have been made in this country, and the difference 
between their results may be accounted for. We have seen that 
the greater the circulation of wind—the greater the amount of air 
passing over water or over moist land—the greater will be the 
evaporation, and, as Mr. Symons cautiously observes : u in the 
middle of a stream,” the position of Mr. Greaves’ gauge, 11 it is said 
