what source the Swedish records of small-pox’ were drawn, 
and whether similar tables could not be obtained for other 
diseases. 
Dr. Farr, with his usual kindness, put me into communi- 
cation with Dr. Berg of Stockholm, who then had charge of 
the Government department of the registration of deaths, 
and he at once sent me a list of the annual death-rates 
throughout Sweden since the year 1774, pa^rticularizing the 
mortality from scarlet fever, measles and whooping cough. 
Dr. Berg accompanied the returns with a letter, which is 
appended to this paper, in which he pointed out the 
possibility of error in diagnosis having been made by the 
earlier Swedish ecclesiastical registrars in the country. 
In this caution, however, I tliink he does less than justice 
to those observers. The tables contain internal evidence of 
accuracy in the characteristic peculiarities of the course of 
each disease, and they bear ample witness to the fact of the 
regular succession of epidemics in distinct cyclical periods. 
I have thrown these tables into the form of diagrams, 
and have supplemented them by similar charts drawn from 
the Begistrar General’s returns, and calculated so as to 
afford a means of comparing the two series of statistics. 
The details of these charts will be more closely studied 
presently, but in the meantime it may be sufficient to 
point out that whooping cough has a cycle of about four 
years, that before the introduction of vaccination, 
constantly i-e-appeared as an epidemic about every five or 
six }mars, that measles prevailed on an average about every 
seven years, and that scarlet fever had an extended period 
of fifteen to twenty years between the great visitations of 
this disease, with lesser undulations of three or four years. 
I venture to bring these charts before the notice of 
this Society (1) because I believe they are unique in 
their kind, and (2) because they afford ground for a dis- 
cussion of the probable causes of the remarkable regularity 
of epidemic cycles. 
