On Irrigation in Tasmania . 163 
tank is large and deep, will be very great; and either 
the slope about the level of the water on the inside must 
be increased to 3 or 4 to 1, or, which is much better, it 
should be made not less than from 2 or 3 to 1, and 
covered with stones or large gravel. It is to be ob¬ 
served, that it is only about the level of the water when 
full that this precaution is necessary ; for when the water 
is low, both the depth and surface being diminished, the 
ripple is greatly reduced, and its effect upon the bank 
trifling. 
The height of the bank must be such as to ensure 
its top being always from 1J to 2 yards above the 
highest permitted rise of the water, both to prevent the 
water running over it in high winds, and also to secure 
it against the water soaking into it. The lower part of 
the embankment will always be sufficiently consolidated 
to prevent the water penetrating, from the weight of the 
superincumbent earth, and, in this colony, from the 
pressure of the feet of the cattle and wheels of carts ; but 
the last yard or two will be more loose and open, and 
therefore more liable to be sodden with the water of the 
tank. I never myself saw but one instance of a bank 
becoming sodden, and in that case all the water entered 
from very near the top. 
To determine the greatest height of the water in the 
tank, it is absolutely necessary that it should be provided 
with a waste channel, of sufficient capacity to carry off all 
the water that can enter the tank in the heaviest rains. 
If the water be permitted to rise to the top of the bank, 
no dimensions or material can save it (unless made of solid 
masonry); but if the water be not permitted to rise too 
near the top, no mischief can happen. 3 he waste channel 
must be made round one end of the bank, so as to lead 
the water quite clear of the artificial earth-work. If 
m 2 
