85 
years, and in some instances, as in 1863, only rising one- 
sixth more than usual. 
In small-pox the difference is not so manifest, except that 
(in Sweden) in the 30 years 1772 to 1802, the waves of this 
disease are about ten times the height of the non-epidemic 
periods, a flood of disease that is only reached in England 
in the great outbreak of 1872. 
2, We may also note the fact that the intervals between 
the epidemics are much shorter in England and Wales in 
the case of both scarlet fever and measles ; and in Sweden, 
with an increased density of the population from 1860 to 
1875, there is also apparent some shortening of the period 
of comparative immunity from scarlet fever. 
It is indeed remarkable how very prevalent this disease 
has become of late years in Sweden. After 1861, it never 
sinks below a point that would have constituted an 
epidemic in former years, and in 1865 and in 1869 and 
1870 it reaches nearly five times the height it had ever 
attained before 1830. 
1 have not seen any explanation of this extreme prevalence 
of the complaint in Sweden in recent times, though I have 
no doubt that it is in some way connected with the greater 
density of the population. The number of persons per 
square mile has just doubled in the course of 100 years. 
1 would point out that an increase to so great an extent is 
not observable in the case of measles, which seems indeed 
to ])ursue the even tenour of its way in the last decade as 
in the first, only a slight increase being noticeable since the 
year 1820. 
Is this because nearly every person takes measles, and 
only a portion of the population need contract scarlet fever ? 
3. We may remark that even in Sweden whooping cough 
and measles reappear as an epidemic after a longer in- 
terval when the previous outbreak had been a severe one. 
Thus, after the years 1793 and 1821, when measles reaches 
