30 
Arterial Drainage 
few inches higher than usual. The water first trickles through, 
the hole gradually enlarges, until, without warning or time lor 
Fig. 3. — Elevation and Section of Sluice witJi P^^P^^'^*'^^"' bank 
TanJcard-lid Door. 
Elevation 
bursts and a whole 
level is inundated, 
and crops and pro- 
perty destroyed to the 
value of many thou- 
sands of pounds. 
In streams dis- 
charging into tidal 
rivers, or on the sea 
shore, doors are ne- 
cessary as a protect- 
ion against the tides. 
The situation of the 
outfall in such cases 
should be chosen 
where the set of the 
tide is on a concave 
shore, and where, con- 
sequently, the water 
is always deepest and 
the outlet the least 
likely to be blocked 
by shoals in dry 
weather. The cill of 
the sluice should be 
placed below the 
level of low-water of 
spring tides, the ex- 
act distance being a 
matter of controversy, 
but two feet may be 
taken as a safe dis- 
tance. 
The simplest form of sluice consists of a wooden tunnel 
made of four 3-inch planks nailed together, with a door or 
clapper at the outer end hung at the top with crooks and 
bands, and falling over the opening of the trunk of dis- 
charge. The door is kept closed .as the tidal water rises and 
presses against it. Cast-iron flanged pipes, bedded in concrete, 
bolted together, with a strip of vulcanised india-rubber between 
the flanges, m;ike a more durable sluice. The door ought to 
be planed smooth, and have a strip of gun-metal inserted in the 
face of the frame against which it shuts. The rod on which 
the door hangs should also be of gun-metal, otherwise it is liable 
Secti 0 N 
