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per cent., or of 5 in 10,000 the mean length of life becomes
for the same death-rates, taken in the same order, 510.54
years, 313.84, 232.86, 187.32, 157.71, 136.73, 121.01, and
108.73.
But the tables further demonstrate clearly, and this
is the point of chief importance, that the health of a population
as measured by the duration of life may under certain
conditions of the birth-rate be as good when the mortality is
high as when the mortality is low. Thus, the mean duration
of life is the same, namely 81 years, with a death-rate of .5
per cent, and a birth-rate of 2.5 per cent., as it is with a
death-rate of 1 per cent, and a birth-rate of 1.5 per cent., or
with a death-rate of 1.5 per cent, and a birth-rate of 1 per
cent.; it is the same, namely 37 years, with a death-rate of
1.5 per cent, and a birth-rate of 4.5 per cent., as it is with a
death-rate of 2 per cent, and a birth-rate of 3.5 per cent., or
a death-rate of 3.5 per cent, and a birth-rate of 2 per cent.;
it is the same, namely 31 years, with a death-rate of 1.5 and
a birth-rate of 6, as it is with a death-rate of 2 and a birthrate
of 5, or with a death-rate of 3 and a birth-rate of 3.5, or
with a death-rate of 3.5 and a birth-rate of 3, or with a
death-rate of 4 and a birth-rate of 2.5 ; it is the same,
namely 58 years, with a death-rate of .5 and a birth-rate of
between 4 and 4.5 per cent., as it is with a death-rate of 4
and a birth-rate of .5. Other examples might be adduced
from the table, but it is unnecessary, inasmuch as a little
consideration will render it apparent that by interpolating
other birth-rates and mean durations of life between those
given in the table we may readily find exactly corresponding
terms under each death-rate column.