182 On Irrigation in Tasmania. 
would be a considerable work ; but the extent of land that 
it would irrigate would be so great, that there is little 
doubt that the advantages to be derived from that and 
navigation together would cause an ample return. 
The objections to this plan are,— 
1st. That it would require a considerable expenditure 
at once. 
2 nd. That it would not much benefit the estates on the 
opposite side of the river, and consequently it would 
be advisable to cut one also on that side; but if it an¬ 
swered on one side, a second one would probably be 
equally advantageous. 
3rd. It must be carried into effect in one complete 
plan,—that is, as respects the navigation,—or it would be 
useless; though, as respects irrigation, if it was only 
carried a part of the distance, the full benefits of that 
part would be obtained. 
The other two modes are, either to cut canals round 
each separate rapid ; or, by removing the stones and rocks 
out of the bed of the river, so far to diminish the current 
as to allow of steamers surmounting it. 
Whichever of these was adopted (but especially if the 
last was approved), the most important step in the plan 
would be to secure a greater supply of water in the river, 
in the summer months, by closing the outlets of the lakes, 
as has been proposed in speaking of irrigation. By this 
means, at a very small expense, an additional depth of 
1 foot or IA feet might be obtained in the river, even in 
its present state. Supposing the lakes at the head of the 
Derwent, Shannon, and Ouse to cover 200 square miles, 
and that the average fall of rain thereabouts is 20 inches, 
it is probable that 2000 million cubic yards of water 
might be retained in them above their natural summer 
level: this would supply a stream in the rapids GO )aids 
broad, and 1 foot deep, for 200 days. To obtain this 
