On Irrigation in Tasmania. 165 
the sluice : this gate may be very small, because, from 
the pressure of the head of the water in the tank, a 
small opening will discharge a great quantity of water. 
Supposing the head of water—that is, the height of the 
surface above the gate—to be 9 feet, the velocity of the 
water through it will be 20,000 yards an hour; and if it 
were required to discharge 10,000 cubic yards an hour, 
or sufficient for 15,000 acres, the gate must be one yard 
high and a foot and a half broad. The interior of the 
sluice must be made of much greater capacity than the 
opening, so that the water in flowing through it may not 
press upon the top and sides, which would be likely to 
burst it. The outer end of the sluice must be secured 
with stone-work, so as to prevent the water in escaping 
from wearing away the earth, and undermining the sluice 
and bank. The accompanying diagrams show the cross 
and longitudinal sections and plan of a bank 6 yards 
high, upon the principles here given. 
Thus the essential points in the construction of an 
embankment are these : — 
1st. That it should have slopes of from 1£ to 2 of 
breadth for 1 of height. 
2nd. That it should not be less than 3 or 4 yards 
broad at the top. 
3rd. That it should be from 1 to 1J yards higher 
than the highest level of the water, and about 2 yards 
higher than the bed of the waste channel. 
4th. That if the tank is large and much exposed, so 
that there may be a heavy ripple upon the bank, it 
should have a slope of 3 to 1 about the highest water 
level, and be covered with stones or large gravel for 1 £ 
or 2 yards above and below that level. 
5th. That it should be provided with an ample waste 
channel, cut in the natural ground, and carried quite 
