The population of a city on April 15, 1915, was 47,175. During the period between March 1 and July 1, 1915, 1,325 new cases of smallpox occurred in the city. Of these cases, 642 persons were ill with smallpox according to surveillance reports on June 1, 1915. The monthly incident rate (per 1,000) of active cases of influence for the 4-month period was: