183 
On Irrigation in Tasmania. 
great supply of water, it would only be necessary to close 
the outlets of 4 or 5 lakes with embankments 2 or 3 yards 
high : the outlets of all lakes are narrow, and the extent 
of work required to close a large one is no greater than 
for a small one. By these works also the dangers of 
floods in the Derwent would be almost entirely removed. 
By this first step more would be effected by a certain 
sum to render the Upper Derwent navigable than ten 
times the sums expended in the river itself. There is a 
system of navigation of vast extent upon this plan in 
Russia : large reservoirs have been formed at the heads 
of the feeders of a main river, throughout a great tract of 
country ; and as there is not sufficient water to keep the 
river supplied constantly, notice is given at certain in¬ 
tervals that an artificial fresh will be formed in it, 
when the boats are collected and loaded in readiness. The 
sluices are then opened at the various reservoirs at such 
times as will allow of the stream from each arriving at 
one point in the main river at a certain time ; and having 
been kept open for some days, so as to allow of the 
boats reaching their destination, they are again closed. 
This alone would probably keep a sufficient depth of 
water in the Derwent, even in the rapids, at all times, 
for such boats as would be most suitable; viz. flat- 
bottomed iron vessels drawing 2 feet water, similar to 
those in use on the Ganges* (but smaller), which, though 
120 feet long, draw only 16 inches with their engines on 
board, and when fully loaded 2 or 2\ feet. But still 
the velocity of the current, though much less than it is 
at present when the river is quite low, would be too 
great for steamers to stem, unless by means of warps 
fixed in such a manner that the boat could get hold of 
them on approaching the rapid, and so by a windlass 
warp herself through. The velocity of the water in 
rapids of small extent is greatest when the river is lowest, 
