e cannot. Thus the dry-bulb line is deflected to the left as
it approaches the surface, the dew-point line to the right. Thus the
effect of the moisture of the ground is to cause the lines to approach.
In the case of grass, fig. 2, the deviation of the dry-bulb line to the
left to form a sharp minimum of temperature at the surface is well
shown. The dew-point line is also shown diverted to the left to the same
point as the dry-bulb; but that could only happen if there were so
copious a condensation from the atmosphere as actually to make the air
drier at the surface than up above. In diagram 1, for soil, the effect
on air temperature and moisture is shown; the two lines converge to cut
at the surface where a dew deposit will be formed. Along the underground
line there must be a gradual creeping of heat and moisture towards the
surface by distillation, the more rapid the greater the temperature
gradient.

The amount of dew deposited is considerable, and, in tropical countries,
is sometimes sufficiently heavy to be collected by gutters and spouts,
but it is not generally regarded as a large percentage of the total
rainfall. Loesche estimates the amount of dew for a single night on the
Loango coast at 3 mm., but the estimate seems a high one. Measurements
go to show that the depth of water corresponding with the aggregate
annual deposit of dew is 1 in. to 1.5 in. near London (G. Dines), 1.2
in. at Munich (Wollny), 0.3 in. at Montpellier (Crova), 1.6 in. at
Tenbury, Worcestershire (Badgley).

With the question of the amount of water collected as dew, that of the
maintenance of "dew ponds" is intimately associated. The name is given
to certain isolated ponds on the upper levels of the chalk downs of the
south of England and elsewhere. Some of these ponds are very ancient, as
the title of a work on _Neolithic Dewponds_ by A. J. and G. Hubbard
indicates. Their name seems to imply the hypothesis that they depend
upon dew and not entirely upon rain for their maintenance as a source of
water supply for cattle, for which they are used. The question has been
discussed a good deal, but not settled; the balance of evidence seems to
be against the view that dew deposits make any important contribution to
the supply of water. The construction of dew ponds is, however, still
practised on traditional lines, and it is said that a new dew pond has
first to be filled artificially. It does not come into existence by the
gradual accumulation