and the Storage of Water. 
27 
in the centre of the barge, and when the spoon reaches the spot 
immediately under this pulley, the chain lifts it vertically until 
it reaches the gunwale of the barge, when the bag is emptied on 
to a shoot and then drawn back again and lowered for a fresh 
supply. The bag is now generally superseded by an iron scoop 
with perforated sides, the bottom being made of steel and hung 
On hinges, and kept in its place by a catch and lever, which is 
released by the man in attendance as soon as it reaches the 
barge ; the bottom then falls, and the contents fall on the shoot 
(Fig. 2). This operation requires four men to work it ; two at 
the windlass, one with the pole, and one to shovel the dirt off 
the shoot into the barge. Under ordinary circumstances, it 
will raise from 25 to 30 tons a day. 
In streams where the current is slow, weeds and aquatic plants 
grow freely on the bottom, and by the friction which they offer 
to the water materially retard its flow. In very irregular and 
shallow channels, much impeded by weeds, the velocity will be 
' reduced from this cause as much as one-half from that which a 
clear course would afford.* In the Spanish irrigation works it 
has been found that the best velocity to keep the channel clear 
of sediment and prevent the too rapid growth of weeds is about 
120 feet per minute, or 1^ mile per hour.j 
In canalised streams of low velocity, where there is no flow 
in summer, or where the movement of the water is very sluggish, 
the weeds require cutting twice and even three times in the year. 
The ordinary method is by joining together several scythe-blades, 
and attaching ropes to each end of the set of knives. Men walk 
on either bank of the stream, and keep drawing the cutters 
backwards and forwards as tlaey move up the stream. The 
weeds are then drawn to shore with rakes and placed above 
flood-level. The cost of this work in the Fen districts, where 
it is termed " roding," is about 20s. per mile for drains from 
15 to 20 feet wide, and 30s. for larger drains. 
* Neville's Hydraulic Tables. 
t ' Irrigation in Spain,' Kcberts. 
