162 
On Irrigation in Tasmania. 
in the smaller rivers, which in their natural state contain 
no water in summer), recourse must be had to the means 
before stated, of storing up the winter rains in the lakes, 
lagoons, and marshes, by closing their outlets with 
earthen or timber dams. 
It may be best here to notice the principal points to 
be considered in forming these reservoirs :— 
First, with respect to the strength, &c. of the em¬ 
bankment when of earth. It may be made of any 
materials (except absolute sand) that may be on the 
spot; only taking care that there is so much more earth 
than stones as to make it solid and water-tight. For its 
dimensions, it must first be noticed, that to resist the mere 
pressure of the water, the weakest earthen bank that can 
be thrown up will have more than sufficient strength. 
No embankment is ever breached by the mere pressure 
of the water : its destruction is always caused by the 
water overtopping it, and so cutting through it; except¬ 
ing, indeed, very rarely by the water soaking into the 
earth of which the embankment is formed, and con¬ 
verting it into half-fluid mud : but even this is caused 
only by allowing the water in the tank to rise too near 
the levcd of the top of the bank. Hence, in determining 
the dimensions of a bank, the points to be considered 
are, what slopes and materials will resist the tendency 
of the earth to slide down from its own gravity, the wear 
of the rain water running down them, and the ripple of 
the water in the tank; and what breadth at top will be 
sufficient to secure the bank from too rapidly wearing away 
by the weather, the tread of cattle, &c. The slopes 
should not on any account have a less inclination than 
1^ in breadth to 1 in height: from that to 2 to I will be 
sufficient to provide against the slipping of the earth, 
and the wear of rain-water in this colony. In very open 
and exposed situations, the effect of the ripple, if the 
