54 
Arterial Drainage 
experimented on will give a general idea of the method of con- 
struction of these valuable reservoirs for water-supply. It is 
situated on the highest ground of the Berkshire Hills, 450 feet 
above mean sea-level, and is excavated in the chalk, A neigh- 
bouring farm, 135 feet below the pond, has to obtain its water 
from a well 108 feet deep. It cannot therefore, from its situa- 
tion, be supplied by springs or surface-drainage. It is feet 
in diameter and 6 feet 8 inches deep, and in shape it resembles 
a shallow rain-gauge, the straight sides meeting nearly in a point 
at the bottom. It is lined with a layer of clay, 12 inches thick, 
mixed with lime to stay the worms, and covered over with first 
a coating of straw to prevent the sun cracking the clay, and 
finally with loose rubble. It was constructed in 1836 at a cost 
of 40/., and up to 1876 had been only once dry, owing to a leak 
caused by the growth of rushes. Exposed to the sun and wind, 
it is liable to great evaporation and loss ; and Mr. Slade contends 
that, theoretically, it cannot derive its source from the con- 
densation of the dew, as the surface of the water heated by the 
sun during the day would be warmer than the surrounding 
atmosphere at night ; and, practically, that it does not do so, 
as his observations show that in no instance did a gain take 
place after sunset, and in the early morning hours occasionally 
a thick mist was observed to rise from the pond's basin and roll 
away over the downs, leaving a strong dew deposition in its 
track. The only source of supply by which the water can be 
maintained in the pond is the rainfall. The total fall for the four 
summer months, June, July, August, and September, was 11'708 
inches. There were in the pond on June 7th, 24,719 gallons of 
water, and there remained on October 2, 18,218 gallons. A 
rainfall of 11*708 inches falling on the area of the surface of the 
pond would represent 23,043 gallons, which, added to the loss 
between June and September of 6501 gallons, would make a 
total to be accounted for of 29,544 gallons. Of this Mr. Slade 
estimates that 6203 gallons were drunk by the sheep at the rate 
of about half a gallon a day each, and the remainder passed 
away by evaporation and absorption of the sides of the pond 
above the water-level. 
Water-power, Water-mills, Weirs, Sfc. — Besides the uses already 
referred to, water is made to serve another purpose as power for 
driving mills and machinery ; and for this end the regulation of 
the supply is all important. It seems an anomaly that such a 
valuable ibrce should be allowed to pass our doors and go away 
to sea, while we are sinking shafts and fetching fuel from the 
bowels of the earth, and transporting it at great expense hundreds 
of miles to perform work, such as driving mills and engines, that 
could as well be worked by water if a regular supply were insured. 
The numerous water-wheels used in the mining-districts for 
