On Irrigation in Tasmania . 
167 
These velocities multiplied by tlie area of the channel 
will give the quantity of water conveyed per hour. 
Hence if a channel is 6 feet wide and 3 deep, the area 
will be 2 square yards ; and if the fall is 1 foot per mile, 
the velocity will be 1000 yards per hour, and the quan¬ 
tity conveyed 1000 x 2=2000 cubic yards per hour, or 
sufficient for about 3000 acres. If the fall allowed in 
the channel be increased, the capacity of it may be dimin¬ 
ished ; and if the fall be diminished, the capacity must be 
increased. By this means the dimensions of a channel 
capable of conveying water for a certain quantity of land 
may be easily calculated. 
It will be evident that, even where the banks of the 
river are not very high, and the soil adjoining not very 
difficult to cut through, there must be a considerable exca¬ 
vation in cutting the upper part of the channel; and this 
may be partly obviated by making a dam across the 
river immediately below the head of the channel. Thus 
if the river is 6 feet deep, and the dam is made 4 feet 
high, the channel may be cut only 3 or 3£ feet deep at 
its head ; and hence it must be calculated in any given 
case which will cause the least expense — to make a dam 
across the river of a certain height, or to cut the head of 
the channel to the full depth of the river. By making a 
dam also, so much additional height in the level of the 
water is obtained, which will bring it sooner to the level 
of the land to be watered. In case of a large river, such 
as the Derwent, the construction of a dam would be too 
expensive a work for a small private channel; but if an 
extensive work was to be undertaken, such as a channel 
of 20 or 30 miles in length, intended to lead water to 
many estates, it might be best to begin with a dam, 
especially as that river furnishes many good sites for such 
works where its bed is rocky. Butin the smaller rivers 
it will generally be advisable to begin with a dam. 
