Trunk Drainar/e. 
57 
side tlio rivci- have always been accustomed to take in large 
quantities of water in summer, through sluices situated lor the 
purpose a few miles below Peterborough, it becomes a serious 
question, and one which gave rise to some opposition of this 
measure, xohat effect the freer admission of the tide xmll have in 
rcndcriiKj the water salt ur brackish hi(jher vp the stream'^ Mr. 
Robert Stephenson, in his Report of 1848, settles this point in a 
very c lear manner ; and the true nature of the tidal operation as 
it affects the supply of fresh water for irrigation or watering of 
cattle is of the greatest importance in many rivers beside the 
Nene. We therefore devote a few lines to the elucidation of 
this subject. By reducing the low-water mark towards the sea, 
or removing impediments to the descending land-waters, we are, 
in effect, increasing the natural fall of the river. This gives the 
down-flowing column of fresh water additional velocity, or, what 
is the same thing, greater mechanical power ; and " the only way 
in which this additional power can be expended, is in driving 
the boundary of the salt water further down the stream.'' The 
improvements in the river all have for their object to augment 
the force and momentum of the fresh water, which will therefore 
overcome the inroad of the salt water at a point nearer to the 
sea ; the boundary between the salt and fresh water, being the 
point where the two hydraulic forces are in equilibrium, will 
necessarily retreat before the stronger force. It is true that 
obstructions in the form of the channel hinder the ingress of 
the sea- flood, and their removal gives it easier admittance ; but 
then the same obstacles impede much more the outflow of the 
freshes ; and by clearing the bed of the river, and adding to its 
capacity, we do not favour each opponent equally with ample 
scope for his strength (which would leave the boundary of 
balanced power where it was before), but we give the greatest 
ainount of play to one party, and bring extra water to his assist- 
ance into the bargain, and so force back the salt water by the 
increased amount of freshes. We have before said that the 
opening of a river channel causes the tide to pulsate further 
inland, and the high or low water mark to traverse more quickly 
up and down the stream ; but then this refers to the level of the 
water, not to the water itself. Like waves in the ocean, which 
travel forward in one direction, although the water composing 
them may be actually running the opposite way, — so the tidal 
swell travels with great speed for a long distance up a river, 
while the salt water, which, by its influx, gave the impulse at 
the lower end of the stream, penetrates slowly only part of the 
way up the channel, receding again for a considerable distance 
before the tide becomes felt at the extreme limit. The more 
inland portions of the river thus have a fresh-water tide, though 
