54 
Trunk Drainarje. 
above tlie meadows, and 2 feet above the quay. Of course the- 
embankments along; the river from its outfall, and along; the land 
side of the " Waslies " or open meadow, must be hig:h and strong to 
hold such a swelling stream from overflovvinff the low contiguous 
fen lands. The amount of rise and fall in the tide is less and less 
the more inland we go. Thus a tide of 22 feet in the open sea 
was at the Nene mouth 21 ft. 6 in. ; at the North Level Sluice, 
16 ft. 10 in. ; below Wisbech Bridge, 13 ft. 7 in. ; above the 
same bridge, 10 ft. 4 in. ; at Guyhirn, 5 ft. 4 in. ; at the Dog- 
and-Doublet, 11 in.; and at Peterborough Bridge, 4 in. Then, 
again, neither high water nor low water is simultaneous along 
the line of the river, but both travel along it ; and, owing to irre- 
gularities in the channel as well as to winds and freshes, they 
are very unequally delayed in different portions of their course. 
Thus it will be high water in the open sea about one hour earlier 
than at Wisbech ; two hours than at Guyhirn, which is only 
one-third the distance further ; four hours earlier than at the 
Dog-and-Doublet, which is little more than one-third the distnnce 
furtlier. With low water it is mucli the same, high and low 
water mark travelling very much slower above Wisbech than 
below. 
The time, duration, amount, and altitude of the tidal pulsa- 
tions at different points of the river course, thus form elements 
in the engineer's calculation of the area of waterway, height and 
strength of banks, &c., required at those points ; and not simply 
the mere inclination of the stream, and the quantity and 
velocity of the waters it must convey. Fears used to be enter- 
tained of admitting the tides to flow far inland, but our modern 
engineers advocate the freest play for both salt and fresh waters 
up and down the rivers. The tide will probably rise a trifle 
higher at Peterborough, when the channel is perfected, than it 
does now ; but it will sink very much lower, that is, the opened 
and deepened channel will give access to a larger tide ; the 
augmented flux and reflux will create an additional and suffi- 
cient scour for clearing the soft river bed, while the low-water 
point will follow more quickly upon low-water at Wisbech, or 
in the sea, and the minor drains will be able to open their 
sluices and emit their water sooner after the tide has begun to 
fall. In a tidal river too there may be a considerable capacity 
of channel at full-water, owing to its width, when at the same 
time, in consequence of its shallowness, there is no adequate 
area of waterway at the ebl>, the only time at which it is in use for 
drainage. This is a most important point in determining the 
requisite dimensions of a river. In the year 1836 the Nene 
areas were as follows : — 
