164 On Irrigation in Tasmania. 
the waste channel is cut through very soft soil, it will 
be necessary to construct a dam across it of rough stone 
or masonry, to prevent the stream from wearing it away, 
or it. may have a work of timber in it for the same end. 
The breadtli of this channel should bear some projior- 
tion to those of the various streams that enter the tank : 
in large reservoirs it may be smaller in proportion than in 
small ones; because, the surface of the water being great, 
it will take long to raise it much, and, as the heaviest 
falls of rain do not continue very long without inter¬ 
mission, they will have ceased before the tank is too 
high, and time is afforded for it to discharge itself by a 
proportionately smaller channel. In most situations in 
the occupied districts of this island, where the rains are 
never very heavy, a channel of from 3 to 6 yards broad 
will generally be found sufficient, supposing the water 
can rise in it to the depth of half a yard or two feet 
without endangering the bank. 
The last point to be attended to in the bank, is the 
sluice for letting out the water when required for irriga¬ 
tion. This must of necessity be made at a low level, in 
order that the whole of the contents of the tank may be 
drawn off. Timber will, of course, be the cheapest 
material in this colony ; but as it will decay, and the 
renewal of it involves the cutting through the embank¬ 
ment to the bottom, (which would be not only expensive 
and troublesome, but, if not carefully and solidly filled 
up again, would endanger the bank,) it would be best to 
form the sluice of rough stone laid in mortar, and 
pointed on the inside with water-cement. 
The masonry should be laid in a channel cut in the 
natural ground, and not in the bank itself, near the 
lowest part of the outlet, and within a foot or two of the 
level of its bed. It may have near the inner end a 
timber gallows in which a gate may slide, so as to close 
