58 ROYAL SOCIETY OF CANADA 
of all kinds from being carried into the air. In so far as fewness 
of micro-organisms is a factor in deciding the hygienic value of the air, 
Winnipeg must be one of the healthiest cities in the world. 
The highest number of micro-organisms found in 10 litres was 
263 on September 28th, 1908. The lowest numbers were 0 in 50 litres 
on March 25th, 1909, and 1 in 75 litres on February 19th, 1909. 
The highest number of micro-organisms falling per square foot 
per minute was 8,506 during a gale on April 13th, 1908. The lowest 
number was 3, which was registered on two occasions, once on January 
7th, 1909, and again on March 25th in the same year. 
Comparative experiments made on the same days on the bare 
prairie at St. Charles, a place six miles from the City of Winnipeg, 
and in the centre of Winnipeg showed that the air in the town was 
about three times richer in micro-organisms than that in the country. 
The average ratio of the number of micro-organisms falling per 
square foot per minute to the number contained in 10 litres of air for 
54 experiments made during the year was found to be 45.48, and the 
ratio varied from 2.9 to 283. From the great variability of the ratio 
we may conclude that the volumetric and plate methods each has 
its own particular and distinct value for estimating the number of 
micro-organisms in the air. A curve plotted from the results given by 
the one method does not coincide with the curve plotted from the re- 
sults given by the other. Variability in external conditions does not 
affect the results given by the two methods in exactly the same manner. 

