92 
On Irrigation in Tasmania. 
the depth of 18 inches, will supply 1 acre of grass land, 
or 2J acres of grain ; and in a tank of which the em¬ 
bankment was 6 yards high, and the depth of water 5 
yards, provision -would he made for irrigating 10 times 
its extent of grass land, or 25 times its extent of "rain. 
The loss by evaporation in this Colony, in broad open 
pieces of water, I judge to be about one-tenth of an inch 
per day, or 15 inches for the whole summer. This I 
found to be the evaporation of Lake Sorell. In India I 
have found the evaporation to be, in an extreme case, 
half an inch per day, when there was a very strong hot 
wind blowing. But, in general, a tank would receive this 
quantity of water from occasional rains during the sum¬ 
mer. The evaporation from the surface of a river will be 
found upon calculation to be quite insignificant. 
There is, perhaps, no part of this subject in which per¬ 
sons not conversant with it are more apt to make absurd 
guesses than in that of sockage. There is no such thing 
as sockage in the bed of a reservoir or river. Supposing 
either of these to have become perfectly dry, a certain 
quantity of water will, of course, as in the case of other 
lands, be required to saturate it; but after this not one 
drop more can be squeezed into it, and therefore there is 
no sockage from day to day : on the contrary, the bed of a 
tank or river being the lowest ground in the neighbour¬ 
hood, the surrounding land, being saturated during the 
winter rains, continues to drain into them during the 
greater part of the summer. This may often be seen ; for 
if a dam is thrown across a river which turns off the 
whole stream, on going a few miles further down a small 
stream will again be found, formed of the drainings from 
the banks. There are, indeed, instances of a small stream 
disappearing in the bed of a channel,, and perhaps re¬ 
appearing at a lower point; but this is a rare circum¬ 
stance. It will also be sometimes found r that a very 
