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8 MORTALITY OF THE CITY OF LONDON.
only is it, that fallacies can be avoided which arise
from the alternate pressure and remittence of epidemic
disease. The havoc effected by a periodical
visitation of influenza, cholera, or plague varies,
in like manner as the ordinary death-rate varies,
in different localities ; and its variation contributes
importantly to fix the healthiness or unhealthiness
of such localities. But obviously, if we wish for
practical purposes to calculate an annual rate of
mortality, and to decide, in respect of any district,
what are the chances of life for its population,
we must distribute the peculiar mortality of the
pestilence-period over those years which intervene
between visitations of the pestilence.
Hitherto, in respect of the City of London, I have
the record of only two years; two years differing
from one another in the proportion of 30 to 22,
and the mean mortality deduced from that biennial
period would be 26 per thousand per annum. I am,
of course, unable to tell you with certainty whether
that ratio be the true average death-rate of the
City; but I incline to believe that an average calculated
from a longer period, with less abrupt fluctuations,
would give a lower figure as the accurate
one.
In future years, so long as I may have the honour
of reporting to the Commission, I purpose proceeding,
step by step, to the construction of a cyclical
average from the materials which will constantly be
increasing; and I trust that many years may elapse