26 
Arterial Drainage 
placed at a level considerably below the drain, the water of 
which they convey, and the end which utters the water being 
placed so much lower than the receiving end as to compensate 
lor the friction of the water in passing through. If placed at a 
dead level, there will be a slight head on the upper side. 
In making new cuts or improving old water-courses, the 
building of new bridges, or the altering of old ones, is a matter 
requiring much attention. The increased rate at which the 
rain-water is now sent to the brooks renders the openings of 
many bridges and culverts, which formerly were large enough, 
of insufficient capacity and therefore a great impediment to the 
flow of the water. The use also of steam-cultivating machines 
and traction-engines, weighing 10 or 12 tons, renders it im- 
perative that bridges should be built both wider and stronger 
than formerly. VVherever practicable, a single arch should 
be used, as offering less obstruction to the flow, and being 
less liable to be blocked by weeds and timber. If the subsoil 
is sufficiently sound, inverts should be avoided, as rendering 
difficult any future deepening of the bed. For the same cause 
it is desirable that the bottom level under the bridges and for 
some distance above and below them should be lower than the 
general gradient of the river-bed. The size of the opening must 
be such as to give a larger sectional area than the cut itself at 
the highest flood-level. 
Improving existing Water-courses. — By removing shoals and 
straightening the stream by cutting off bends, the discharge of 
the flood-waters will be increased in proportion to the increased 
water-way, the diminished distance the water has to travel, and 
the smaller amount of friction from the sides and bottom which 
it has to encounter. Improvements of this nature should always 
be commenced at the outfall, otherwise the water will be sent 
into the lower reaches of the stream with such increased rapidity 
that the unimproved portion will not be able to discharge the 
water poured into it with sufficient velocity, and flooding of the 
lands at the lower levels will ensue. 
The removal of shoals is effected either by hand-labour, the 
water being dammed up in sections by sheet-pile dams ; or, when 
this is not practicable, by dredging with a small barge and spoon 
and bag. The spoon is formed with a ring of iron, about 2 feet 
in diameter, having a bag of strong leather attached to it by 
leather thongs. The ring is attached to a pole which is lowered 
from the end of the barge to the bottom of the stream. A chain 
made fast to the ring is wound up by a windlass, fixed at the 
other end of the barge, and the spoon is thus dragged along the 
bottom, being guided in its progress by the man who holds the 
pole. The chain passes over a pulley suspended on an upright 
