184 On Irrigation in Tasmania . 
aiul diminishes as the river rises; and hence, by keeping 
a greater supply in the river, the velocity will be di¬ 
minished. 
The expense of avoiding the currents in the rapids by 
cutting channels round them, would, in most of those of 
the Derwent, not be great. Supposing the total fall in 
the rapid to be 3 feet,—and they do not appear to exceed 
that in general,—a canal might be cut a quarter of a mile 
long ; and if the water in it was 2A feet deep, the velocity 
would then be about 2J miles per hour, which could 
easily be surmounted. If the cut was made only 200 
yards long, with a fall of 3 feet, and depth of water of 2| 
feet, the current would be about 3J miles per hour, which 
would not be too much. At most of the rapids there 
are very convenient places on one side of the river or 
the other for making such cuts, where it would neither 
be very expensive to dig them, nor would they be much 
exposed to injury in floods. The navigation of the river 
might be effected by this means, even without an addi¬ 
tional supply of water being secured in the lakes, but 
probably at a much greater expense. 
The third means for passing the rapids is to blast and 
remove the rock in the bed of the river for a moderate 
breadth, so as to form a rough canal along the centre of 
the channel,—or, if more convenient, near one side,—so 
that, instead of a succession of sudden falls, there might 
be a gradual descent; and the greater the length for 
which this was done, the more gentle the current would 
be. Which of those two last plans might be best, would 
be determined only by a particular examination; and 
probably one might be best in some rapids, and the other 
in the rest. In the course of the river there may be two 
or three rapids which would require considerable labour 
and expense, but in general this would not be the case. 
The work, whether in the channel of the river or by the 
