134 
J. HOPKINSON-WATER AND WATER-SUPPLY 
state that the moon is now in; its surface and atmosphere will he 
entirely deprived of water, and eventually its atmosphere also will 
he dissipated. 
The second cause is purely a geological one. The higher the 
land, the greater is the rainfall, and we know that denudation, 
where the counteracting effects of upheaval have been absent, must 
have greatly lessened the height of our hills and deepened our valleys. 
On the Chalk, as in Hertfordshire, although the wasting effects of 
sub-aerial denudation are imperceptible to us, there is no less surely 
a gradual lowering of the surface of the ground. 
The third cause of decreasing rainfall, which has only been 
operating in comparatively recent times, is the disappearance of 
forests. There can be little doubt but that the greater part of 
Hertfordshire has within historic times been one vast forest. In 
the reign of Edward the Confessor the country between London and 
St. Albans, “as well as all parts of the Chiltern,” is said to have 
been “covered with thick woods and groves.”^ Even since the 
county was first mapped by our Ordnance surveyors, the Hatfield 
'Woodhall woods have been converted into arable land ; and as trees 
anywhere in England now die, how seldom it is that others are 
planted in their place ! 
These causes of decreasing rainfall, all of which are in operation 
at the present time, are so slow in their action that their results 
are inappreciable to us; hut that the rainfall has at one time been 
much greater than it is now, the existence all over the world of 
dry valleys and river-beds affords sure evidence. Dr. Livingstone 
was much impressed with the evidences of the desiccation of the 
southern portion of Central Africa. One large tract between the 
Orange River and Lake Ngami, he says, “has been called a desert 
simply because it contains no running water and very little water 
in wells. ... It is remarkably flat, hut intersected in various 
parts by the beds of ancient rivers.” He traversed a high road 
“ which lies generally in the bed of an ancient river or wady that 
must formerly have flowed north to south.” And again he says of 
this desert: “ The soil is sandy and there are here and there indi¬ 
cations that at spots which now afford no water whatever, there were 
formerly wells and cattle stations.”! A similar account has been 
given of Northern Africa. In north central Asia, the great desert of 
Gobi, a sand-waste from half to three-quarters of a million of miles 
in area, was not always the sterile, waterless, and treeless region it 
now is. It was formerly the seat of many large and flourishing 
cities, and must therefore have been fairly well watered; but the 
native Mongols cut down the trees, the rainfall decreased, and the 
sand advanced and buried the cities. The Dead Sea was at one time 
150 miles in length, and its waters stood 1300 feet higher than 
they do now, this diminution in extent and depth being due to a 
decreasing rainfall; for there are waterless gorges over a thousand 
feet in depth leading into it, and down them powerful rivers must 
* Newcome’s ‘ History of the Abbey of St. Alban,’ pp. 38, 39. 
t ‘ Missionary Travels and Besearches in South Africa,’ pp. 47, 53, 54. 
