164 OBSERVATIONS ON THE FALL OF RAIN. 
conduits by which the rain which falls is rapidly 
conveyed to the contiguous streams, and the same 
quantity of water which formerly required several 
days to make its escape, now passes off in a few 
hours. Some quiet swamp or bog at the head of 
ariver, which had for centuries contributed to 
regulate the quantity of water in the stream, by 
absorbing the superabundance of wet periods and 
gradually discharging it at other times, becomes 
allotted off in farms, trenched in all directions 
and thoroughly drained. The water is then no 
longer retained to assist the streams in dry pe- 
riods, but runs off to swell the floods as quickly 
as the drains can be made to carry it. 
This increasing irregularity, combined with a 
growing demand for further power, has contributed 
to the construction on many streams, of large 
reservoirs for the purpose of impounding the sur- 
plus water of wet seasons to be applied in sup- 
plying the wants of periods of drought, and by 
such artificial means to regulate the quantity of 
water in the stream. 
Wherever these reservoirs have been con- 
structed of sufficient capacity, opportunities have 
been created for ascertaining what proportion the 
