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without result, it does not follow that he is insusceptible to
small pox, as it frequently happens that when re.vaccination
has failed, say at 20 years of age, it succeeds at 25, or some
later period of life.

This table shows that in proportion to the ages of the
vaccinated population, the liability to an attack of small pox
increases up to about 30 years of age, and after that
decreases. The questions then arise,—was vaccination more
effectually performed, or was the lymph of better protective
power 30 years ago and previously to that time, or is there a
larger proportion of re-vaccinated persons amongst those
who are more than 30 years, than amongst younger persons?
There is however another proposition, viz.,—do persons
become less susceptible to this disease as age advances ?
I must leave the answers to these questions for others to
solve. I have mentioned 30 years as the probable age at
which fewer persons are proportionately attacked, but it may
be later, as at 25.45 years of age the proportion of cases is
nearly the same as the percentage of population.
I have already mentioned that there were 90 cases
amongst unvaccinated persons, against 602 said to have been
vaccinated. This seems to be a very large number when we
consider that it is not probable that there are more, if so
many as 5 per cent. of the entire population, who have not
either been vaccinated or had small pox. If this assumption
be correct, there would be a little more than 9,000 persons
in the Hackney District, who were unvaccinated, against
177,000 who were vaccinated, which would give about 1 per
cent. of attacks amongst the unvaccinated, and 0.34 per