77 
always an increase in the number of persons affected. It 
is evident therefore that the complaint has a true two years’ 
cycle. 
I have noted the same regularity in returns of the 
disease made in St. Marylehone, London, by Dr. Whitmore. 
In the same returns of disease there are indications of 
the observance of regular periods by other epidemic 
diseases, but the records as yet extend over too brief a 
time to show the fact with certainty. 
The Registrar General’s returns of deaths also present 
some evidence of a rhythmical succession of epidemics of 
such diseases as small-pox, scarlet fever, measles, and 
whooping cough ; but the causes of death have only been 
given to the public since 1848. 
Dr. Farr kindly sent me further returns for the five years 
1838 to 1842, but, even with this addition, the area of 
observation is too short to permit of a satisfactory 
generalisation. I have, however, traced out diagrams 
giving the information to be gathered from these sources. 
But in Mr. Simon’s admirable papers relating to the 
History and Practice of Yaccination, presented to the 
General Board of Health in 1857, there is a diagram 
(which I have copied and placed before you) showing the 
small-pox death rates in the kingdom of Sweden for 100 
years — from 1749 to 1849, inclusive.^' 
This diagram, though it is only used by Mr. Simon as an 
evidence of the protective influence of vaccination, shows 
distinctly the regularity of the small-pox cycle up to the 
year 1800, when vaccination was fully carried out. 
A period of 100 years gives a fair basis for an indication. 
It therefore occurred to me to enquire from Dr. Farr from 
^Although it is beside my present purpose, may I say incidentally 
that the chart in question proves to demonstration the wonderful pro- 
tective power of thorough vaccination, and that I cannot understand 
how any one with this evidence before him can for a moment doubt its 
beneficent operation. 
