79 
It is indeed unnecessary, with these charts before us, to 
enter into evidence to prove the existence of periodic times 
in epidemics, and we may perhaps be allowed at once to 
pass on to the modes in which such phenomena may be 
supposed to be brought about. 
1. It is natural in the first instance to attempt to connect 
the epidemic cycle with some other coincident recurrent 
events. As Prof. ''Jevons says, in speaking of the principle 
of forced vibrations (Principles of Science, vol. ii., page 66), 
“ Whenever we find two phenomena which do proceed, 
time after time, through changes of exactly the same 
period, there is much probability that they are connected.” 
I have already pointed out in several papers that there 
is undoubtedly some favouring influence exerted by seasons 
upon the different epidemics. Even in non-epidemic years 
there is an evident tendency for these complaints to rise 
at that season in which other affections of the organs most 
attacked by the special epidemic disease are most common. 
Whooping cough and measles in the winter and spring, 
scarlet fever in the autumn, when sore throat is most com- 
mon, continued fever also towards autumn, when derangfe- 
ment of the liver and bowels are frequent. 
But it is impossible to discover any ordinary recurrent 
meteorological element that will account for the cycles of 
any one of these disorders. Atmospheric changes recur an- 
nually, but epidemics come every two, four, six, or more 
years. 
There is indeed one interval that might possibly ha,ve 
some influence either directly or indirectly upon measles, 
small-pox, or scarlet fever that have an approximate five or 
six year period, and this is the well-known sun-spot period 
of astronomers. 
I have obtained from different sources records of these 
occurrences since the year 177*3, and have drawn out 
charts of the periods in a fashion similar to that adopted 
