58 
Trunk Drainage. 
this is at first set in motion by the pressure of an inflowing- flood 
of salt water. The sea water, though extending a far less dis- 
tance than the rise and fall of the stream, contaminates the river 
beyond the point of its complete repulsion ; at first undermining 
the fresh, because of its greater specific gravity, it mingles with 
it in its advance, so that it is a matter of observation to ascertain 
how far up the river the waters may be found blended. Before 
the completion of the Nene Outfall the brackisli water readied 
up to the Dog and Doublet, in very extreme cases to Peter- 
borough, but subsequently its boundary has receded to Guytiirn ; 
and the experience of similar improvements in the river Clyde is 
evidence in the same direction. Sir Joim Rennie's experiments 
in 1836, made upon the Hundred Foot, or Ouse river, show that 
the fresh water is not sensibly affected by the salt at neap tides,. 
6 miles above Lynn ; but at spring tides, at hig^h-water, it was a 
little brackish about 14 miles above Lynn. At a distance of 18 
miles above Lynn harbour there is a tunnel through the bank of 
the river to take water into the fen for supplying the cattle, 
beside several others higher up the stream ; and the water is 
certainly fresh there or it would not be used for such a purpose. 
The points on the Nene, where water is required, are several 
miles further from the sea than is the above-mentioned tunnel, so 
that no fears need be felt by the adjacent farmers on this head. 
And as the brack-water line has been already drawn backward 
by tlie outfall improvements, it is evident that the present com- 
pletion of the channel will not be likely to send the fresh-water 
boundary further from Wisbech. 
The greatest obstructions to the river waters, as we have seen, 
exist in the toicn of Wishech ; and it has been deemed sufficient 
for the purpose, as well as of less expense, to augment the water- 
way in its present circuitous course rather than excavate a new 
straight cut on one side of the town. The Wisbech Corporation 
strenuously opposed the formation of such a cut and the con- 
verting of their harbour into a wet dock, just as they have 
always struggled to prevent any radical improvement of their 
miserable river as a safer berth for larger shipping, though ob- 
viously the best means of swelling tlieir commerce and Ijenefiting 
their town. Heavy as the works must necessarily be, and costly 
as the purchase of valuable buildings proves, it has been decided 
to deepen and widen the channel through the town ; to take away 
the present stone bridge, with its single arch of 65 feet span, and 
at a cost of about 8000/. erect a new one, of 86 feet watcr-wa3', 
on a better site ; (this may be made an opening bridge at any 
time by order of tlie Admiralty ;) also to pull down certain 
houses which now occasion a rectangular bend in the stream ; to 
Aviden the channel by altering the banks, buildings, and gra- 
