these figures by 43.71 in ft of road frontage per house, we
find that the effective width of the impermeable roadway is
increased by 6 ft 10 in for the front portions of each house,
and by a width of 8 ft 7 in, for the back portions, making a
total width of 36 ft + 2(6 ft 10 in) + 2(8 ft 7 in) = 66 ft 10
in, say 67 ft On this basis the impermeable area in the town
therefore equals: 43,7841 in ft x 67 ft =2,933,528; and 30,766
lin ft x 36 ft = 1,107,576.

Total, 4,041,104 sq. ft, or 92.77 acres. As the population is
10,000 the impermeable area equals 404, say, 400 sq. ft per
head, or ~ (92.77 x 100) / 2037 = 4.5 per cent, of the whole
area of the town.

It must be remembered that when rain continues for long
periods, ground which in the ordinary way would generally be
considered permeable becomes soaked and eventually becomes more
or less impermeable. Mr. D. E. Lloyd-Davies, M.Inst.C.E., gives
two very interesting diagrams in the paper previously referred
to, which show the average percentage of effective impermeable
area according to the population per acre. This information,
which is applicable more to large towns, has been embodied in
Fig. 16, from which it will be seen that, for storms of short
duration, the proportion of impervious areas equals 5 per cent.
with a population of 4.9 per acre, which is a very close
approximation to the 4.5 per cent. obtained in the example just
described.

Where the houses are scattered at long intervals along a road
the better way to arrive at an estimate of the quantity of