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to show in what degrees the epidemic diseases, which have prevailed
in London, have prevailed in them.
As a preliminary step to this inquiry, it is necessary for us
to have a clear understanding of the principles on which the
determination of the death-rate depends. A death-rate is in
principle the ratio between the number of individuals forming
a population, and the total number of deaths which have
occurred in that population during some given period of time,
ordinarily for convenience fixed at twelve months. Thus if,
out of a population of 10,000 persons, 200 die in the course
of a year, two deaths occur to each hundred of that population,
and the death-rate is 2.0 per cent. But practically
while the number of deaths occuring in any locality during
a year is a definite number which can with little difficulty
be accurately ascertained, the other element in the calculation
is a varying number which can only be approximatively
estimated. Populations of towns, of districts, of parishes are
constantly shifting and undergoing other changes. Hence,
for the purposes of the death-rate, not the total number of
different persons who have resided in the place during the year,
but the mean average number of yearly residents has to be
taken. It is manifest that if there be a shifting population in
a locality, but the actual number of persons present never undergoes
any variation, the death-rate may be as easily and as
simply ascertained as if the population had been perfectly
stationary. But if the population changes not only in respect
of individuals, but in respect of numbers also, it is manifest
that unless these latter changes can be reduced to some law,
neither the total nor the average population can be ascertained,
and the determination of the death-rate becomes an impossibility.
The population of England is counted once every ten years,
and from the results of these numberings (which take place in
April) the mean average population both of the census-years