MEANS OF COMMUNICATION. 03 
attached to the mitre-post is wound upon a drum by means of cranks and 
cog-wheels, and the gate thus drawn into its open position. At each end 
and upon each side of the chamber a recess is left in the wall for the recep- 
tion of the gate when opened (pl. 10, jig. 21), allowing it to form an 
uninterrupted line with the wall, leaving no projection against which the 
boat may strike in passing. The most common method of opening the gate 
is by means of a heavy beam, termed the balance beam, because it assists 
in balancing the weight of the leaf upon the gudgeon and keeping the quoin 
in an unstrained position. A lock-gate with a balance beam is represented 
in pl. 10, jig. 2. The balance-beam is mortised upon the mitre-post and 
quoin, and extends some distance out upon the bank; in many cases an 
additional weight is put upon the end of the beam when it is not sufficiently 
heavy. When the gate is to be opened it is backed through the water into 
the recess by a steady push against the end of the balance-beam. 
A lock-gate ofa different construction from that above described is repre- ~ 
sented on pl. 10, jigs. 3b and 4. The gate here consists of a single leaf, a, 
and instead of turning on a pivot, it slides into a lateral recess, being moved 
by means of the windlass d, and ropes which pass over the pulleys } and e. 
This method of construction is not to be recommended, and is rarely met with. 
P1.10, jig. 5, shows a kind of drop-gate, which may be used in small 
canals; on the left is a side view with open gates, on the right a front view 
with closed gates ; the drop-gate may describe a quadrant, and be opened 
and closed without trouble. These doors have not been found very prac- 
ticable. 
The valves through which the water enters and leaves the chamber are 
small doors, made either in the side of the chamber-wall, the top of the 
end-walls, or in the lock-gate. In the former case a conduit must be built 
by the side of the lock for the discharge of the water, as seen in pl. 10, figs. 
27 and 28. It is usual to have the valves in the gates. They must be so 
constructed as to be readily opened and closed, and to be water-tight when 
closed. Those generaliy employed are either slide-valves or paddle-~walves. 
The slide-valves may move vertically or horizontally; the vertical slide- 
valve is the simplest in its arrangement, and is therefore most frequently 
used. It moves in grooves, and is opened and closed by means of a rod 
which passes up to the top of the gate, and is raised or lowered by a screw, 
or arack and pinion. /. 10, jig. 22, shows a vertical slide-valve in the 
gate, and jig. 28 a similar one in the side wall of the chamber. The hori- 
zontal slide-valve is moved in a similar manner, but slides horizontally, 
which may be effected by means of a rack on the side of the valve, driven 
by a pinion on a vertical axis projecting above the water, and turned by a 
erank. The paddle-valve is one which turns about its middle, or at one 
side on a vertical axis which reaches up to the top of the gate, and is turned 
by means of a crank. The paddle-valve is neither as safe nor as easily 
worked as the slide-valve. The size of the valves is dependent upon the 
quantity of water to be discharged in a given time, and upon the head of 
water; they vary from 20 inches to 4 feet square, and are generally made 
of cast-iron. 
633 
