On the Annual Water-discharge of Large Rivers. 261 
The total annual water-discharge of the Ganges-Brahmapttra 
system may be estimated, therefore, at 288°95 cubic miles.* 
Sir John Herschel; states (without quoting his authority) that 
the Irawady delivers, on the average of the whole year, into the 
sea 35,000 cubic feet per second ; of which gspooth part by weight 
is silt. This amounts to an 
Annual Water 
Discharge of > =75:038 cubic miles (statute). 
Trawady, 
Mr. Everest found that, during the rainy season, the mud held 
in suspension by the water of the Ganges at Ghazipir amounted, 
on the average, to ;3,th part by weight of the water-discharge. 
If we neglect the mud carried down by the the river during 
the other eight months of the year, we can find a limit to the 
rate at which the rain-basin of the Ganges is being reduced by 
the action of the rainfall. 
During the four rainy months the quantity of water passing 
Ghazipitr is 
494208 x 60 x 60 x 24 x 30 x 4 cubic feet. 
Reducing to cubic fathoms, and multiplying by 6, to reduce to 
tons,t and dividing by 428, to find the weight of mud, we find 
494208 x 60 x 60 x 24x 30xK4x6__ 
63 x 428 
332,550,000 tons of mud.§ 
We can now estimate the number of years required to scrape 
off one foot from the whole surface of the Gangetic-rain-basiu 
above Ghaziptr, by means of the formula 
2,660,000 x A 
a= ice ae. 
* This is much greater than the discharge of the Mississipi-Missouri system. 
t+ Physical Geography, p. 207: Edinburgh (1869). 
{ A cubic fathom of water is six tons q. p. 
§ Sir John Herschel (without quoting his authorily) gives 534,600,000 tons as the 
sea-discharge of the Ganges. 
|| This formula is readily deduced from the following facts :— 
1. The mean specific gravity of surface rock is 2°66, 
2. A cubic fathom of water weighs 6 tons. 
3. A geographical square mile contains 1,000,000 square fathoms. 
Scien. Proc., R.D.S. Vou. 1., Pr. tv. T 2 
