1905-6.] Studies in Immunity : Theory of an Epidemic. 491 
probably to the comparative exhaustion of susceptible persons. 
The statistics refer in this instance to the deaths alone. An epi- 
demic occurring in Warrington* in 1743 (diagram VII., Table A, 
Ho. 8) is given for comparison. The asymmetry of this is not so 
great. Although both these epidemics are definitely limited as to 
beginning and end, the constants are still required by type IV. 
This great asymmetry was not the rule, however, in places where 
smallpox was more frequently epidemic, and where, in consequence, 
a sufficient dilution of the susceptible persons existed to allow the 
epidemic to run a more natural course, and when even at the end 
there were still present in the population sufficient persons open 
Each abscissal unit is one month. 
to infection to permit the decay in the infectivity of the organism 
to be observed. An epidemic in Glasgow in 1784 f is given to 
illustrate this (Table A, Ho. 10). 
The epidemics of smallpox in Gloucester t in 1896 and London § 
in 1902 (Table A, Hos. 11, 12, 13, 14) may be compared with 
those of last century. At first sight, in epidemics where all the 
machinery of modern sanitation has been brought to bear, it might be 
expected that the form of the course would in some way be altered. 
On examination, however, the course of the London epidemic is 
* Report on Epidemic of Smallpox in Warrington in 1892-3, p. 7. 
t Watt’s Treatise on the Chincough , Glasg., 1813, p. 344. 
t Report of the Royal Commission on Vaccination, — Appendix on Gloucester. 
§ Reports of Metropolitan Asylum Boards, 1901-2. 
