166 
On Irrigation in Tasmania . 
clear of the artificial bank, being protected by stone or 
timber work across it in one part, if the soil is soft. Its 
bed should be 1J or 2 feet below the highest level to which 
the water is allowed to rise, or about 2 yards below the 
top of the bank. 
6th. A sluice of masonry must be laid in the natural 
soil, and furnished with a timber sluice-gate sufficiently 
large to discharge the greatest quantity of water that 
can be required at one time. 
When the river is either naturally or by means of such 
reservoirs provided with a sufficient stream of water, it 
remains to lead it off by means of channels to the lands 
to be watered ; which channels for that purpose must have 
a less fall than the river itself has. Thus if the river has a 
fall of 6 feet per mile, and the land to be watered is 15 feet 
above the bed of the river, the head of the channel must 
be situated 3 miles higher up the river than the land to 
be watered, and it may then have a fall of 1 foot per mile, 
which will generally be found a convenient fall for such 
channels. The velocity of the current in rivers and 
channels depends upon these two things—the depth 
of water, and the fall per mile; and in such channels it 
may be taken to be as follows :— 
Fall of 1 foot per mile 1 foot deep 600 yards per hour, 
2 . 1 . 850 
3 . 1 . 1000 
1 . 2 . 850 
2 . 2 . 1200 
3 . 2 . 1400 
1 . 3 . 1000 
2 . 3 . 1400 
3 .3 . 1800 
