BY A. MAULT. 189 
screened so that all floating matter with a greater dimension than half 
an inch will be kept back and not discharged into the Derwent at all. 
In the second place, as to the recipient of this discharge, it is diluted 
sea water, the volume of which, in what may be considered the imme- 
diate basin of discharge, is about 60,000,000 tons ; one fifteenth part of 
which, say four million tons, is, on an average, displaced and replaced 
every tide; and the dilution of which is caused by a mean daily inflow 
of about 16 million tons of land water. In the third place, as to the 
point of discharge, it is into the tideway—that is into a place where it 
will be directly affected and acted upon by the displacement and replace- 
ment caused by every tide, and by the inflow of land water, the joint 
forces of which will secure the commixture of the matter discharged 
with the recipient water, and also its downward flow towards the sea, 
In connection with the second and third of the above-mentioned con- 
ditions, it should be mentioned that tidal action, though varying 
throughout a lunation, may be taken to be regular with regard to the 
lunations throughout a year. On the other hand, the inflow of land 
water varies greatly from the mean quantity stated. In April, 1884, 
in connection with work at Meadow Banks, the Hon. N. J. Brown told 
me that the Derwent was lower than either he or his manager had ever 
noticed it. 1 estimated that there was then passing 1,925,000 tons of 
water a day, say one-eighth of the mean daily flow. As this quantity 
is deduced from gaugings above the mouths of the Russell Falls, Styx, 
Plenty, Lachlan, Sorell, and Jordan Rivers, and numerous smaller 
streams, I think it safe to say that the minimum flow of land water past 
Hobart is not less than 2,000,000 tons a day. On the 28rd September 
in that year there was the greatest flood I ever saw in the Derwent, and 
ina paper I read before the Royal Society in that year, I find that I 
estimated from such calculations as were possible in the case, that 
216,000,000 tons of water passed New Norfolk that day. As higher 
floods have been known, probably more than twelve times the mean 
daily quantity of land water sometimes passes Hobart in 24 hours. 
But whatever may be the fluctuations in the daily flow, itis certain that 
this daily flow must be of immense volume, as it represents the 
drainage of about 1,400,090 acres of ‘‘ wet” area with a mean rainfall of 
more than 55in., and of more than a miliion acres of ** dry” area with 
a mean rainfall of more than 20in. The consequent dilution of tidal 
water is clearly shown in the analysis given further on. 
Taking into consideration the facts above given in connection with 
the conditions stated, the following deductions are evident :—l. That 
the noxious matter to be discharged is infinitesimally small in quantity 
when compared with the volume of water into which it falls. Taking 
the volume of the basin immediately in front of Hobart, the daily pro- 
portion would be less than one part in 28 million parts. Supposing the 
water of this basin tobe drinking water, the addition of this quantity 
' of polluting matter would take several months to reduce it, even if it 
were not being renewed, below the standard of purity for potable water 
as fixed by the Rivers Pollution Commissioners in England; conse- 
quently, as it is not drinking water, its addition cannot appreciably 
affect the Derwent, 
2. That as none of the matter to be discharged will be floating matter 
of the character described as being cast ashore by winds or currents at 
Sandy Bay, the fact that such matter is there cast ashore has nothing 
to do with the question of what. will become of the thin liquid sewage 
that will be discharged at Macquarie Point. If this does not affect the 
water of the basin into which it is to be immediately discharged, it 
certainly cannot affect the water of Sandy Bay still turther off. 
These are not merely theoretical deductions, but practical ones, of 
which the truth is capable of proof, and is being proved every day at 
Hobart. The Hobart Rivulet is, as everybody knows, in fact, and by 
