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of death rates do not give more than approximate indications of the net gain or loss to the community
by fluctuations of mortality, and it is in arriving at a more accurate result in this respect that a life
table is especially useful. The following example of an extreme case may show this more clearly.
Assume that in a certain community 800 deaths occur in one year and that 400 of these deaths
occur among persons aged 0—25 whose mean future lifetime, is say 30 years and that the remaining
400 deaths occur among persons aged 25 and upwards, whose mean future lifetime is say 15 years,
then these 800 deaths will represent a total of 18,000 years of "life capital" lost to the community
for—
(400 x 30) + (400 x 15) = 18,000 years.
Now suppose that 700 deaths occur in the same community in the following year, and that
600 of these deaths occur among persons aged 0—25 and the remaining 100 among persons aged 25
and upwards then the loss of "life capital " to the community will be (600 X 30) + (100 X 15) =
19.500 years. Thus, while there is a saving ot 100 lives in the second year compared with the first,
there is an actual loss to the community of l,500years of " life capital."
In order to apply the life table to the calculation of "life capital" it is necessary to ascertain
the mean future lifetime of the population living at each age period. The life table shows the
expectation of life of persons at the exact age x, but it will easily be seen that the expectation of life

This table furnishes an accurate means of comparing the effect of fluctuations of mortality at
different periods in the London population, for if the number of deaths which would have occurred at
the rates of 1891.1900 in each age-group for any period be calculated and compared with the deaths
which actually occurred during the period, the differences between the two sets of figures will be the
number of lives saved or lost by fluctuations of mortality in the given period, and by applying to
these numbers the figures shown in the table the gain or loss may be expressed in terms of "life
capital." In my annual reports for the years 1894 onwards, I have adopted this method of comparison
on the basis of the experience of mortality in London daring the decennium 1881-90, in the same
manner as shown by Dr. Tatham in a report on " The Health of Greater Manchester during the three
years, 1891-3."
The figures shown in the table may also be utilised for the purpose of estimating the total " life
capital" of the London population at any period, and the result of this calculation is shown in the
following table. The population figures represent the estimated mean population living in London
during the decennium 1891-1900.