257 



average fall of the river along so mucli of its course as I am 

 acquainted with above tidal influence is rather more than 

 4 feet to the mile, representing 2,000 horse-power to the 

 mile, much of which could be made available or transformed 

 into electric dynamic force. 



Again, as far as can be judged from the Government 

 charts, the area of the basin drained by the Derwent above 

 New Norfolk is about 3,200 square miles, or a little more 

 than two million acres. If it be assumed that there is a 

 mean yearly rainfall of 36 inches over this area, of which 

 one-half is taken up by evaporation and plant nourishment, 

 and the other half drained off by the river, the river at 

 New Norfolk would have to carry off a mean daily quantity 

 of 13 J million cubic yards, or 10 j million tons of water — a 

 quantity about equal to that of the foregoing calculation of 

 v/hat is actually carried off. 



Eesidents in the valley above New Norfolk say that the 

 flood of the 23rd of last month was higher than any seen 

 since the great flood of 1863. Below New Norfolk the water is 

 said to have risen higher about four years ago, when perhaps 

 the flood coincided with exceptionally high tides in the river. 

 But both above and below the town mentioned the flood of 1863 

 was much higher. At Yalleyfield they say that it was about 

 4 feet higher than that of last month, at Mr. Inge's about 

 3 feet, and at the mouth of the Plenty Eiver about 2| feet. 



I did not see the river when the flood was at its highest on 

 the 23rd September ; but on the 24th, when I examined it, 

 the water had gone down several feet, still the marks of its 

 greatest height were easily to be seen. At the mouth of the 

 Dry Creek rivulet this greatest height was about 17 h feet 

 above the mean level of my former observations, and at the 

 mouth of the Plenty Eiver nearly 20 feet. This variation is 

 to be accounted for by the difference in the conformation of 

 the river at these two points as regards direction, inclination, 

 and obstructions. I had not the opportunity of observing 

 the flood higher up the river. 



Although possessed of the sectional area of the flood- 

 water and many other elements necessary for calculating the 

 quantity of it that was passing, it is quite impossible to 

 make anything but a mere approximation as to this quantity. 

 There are so many eddies and swirls, so much still water, so 

 much back-draft and under-draft, that only a guess can be 

 made as to the mean hydraulic depth and the rate of the 

 real downward current of water. If a calculation be per- 

 missible in such circumstances I should say that at the 

 height of the flood on the 23rd September about nine 

 million tons of water were passing every hour, about 21 

 times the mean hourlv flow of 430,000 tons. 



