[BULLER & LOWE] MICRO-ORGANISMS IN THE AIR OF WINNIPEG 57 
dust and dirt which are inevitably associated with the streets of a big 
city, where street-cars, automobiles and other agencies keep the air in 
constant motion and full of suspended particles. 
COMPARISON OF THE VOLUMETRIC AND PLATE METHODS. 
In the course of the year’s work altogether, 57 double experiments 
were made with the volumetric and plate methods on the same days. 
The plates were exposed immediately after the aspiration of 10 or 
more litres’of air through the experimental tube. The ratio of the 
number of micro-organisms falling per square foot per minute to the 
number contained in 10 litres is given in the fourth column of the Table. 
The average ratio of 54 of the experiments' was found to be 45.48, 
and the ratio varied from 2.9 to 283. 
The great variability in the ratio goes to show that the two methods 
have different values for estimating the number of micro-organisms 
in the air. The results given by one method are affected by changes 
in the weather in a different manner to the results given by the other 
method. Thus, during strong gales, the ratio tends to reach its maxi- 
mum, as is shown by the data given in the Table for April 13th, 1908, 
and April 24th, 1909. The lowest ratio, namely 2.9, on September 
28th, 1908, occurred when the number in 10 litres of air reached its 
highest value and when the wind was only moderate. In general, it 
is not easy to trace the exact causes of the variations in the ratio. 
SUMMARY. 
The number of micro-organisms in the air of Winnipeg varies from 
week to week throughout the year. The variations depend, as in other 
localities, upon temperature, wind, and the fall of rain and snow. 
The average number of mic1o-organisms in 10 litres of air, as the 
result of 52 weekly observations, was found to be 24. So far as the 
organic contents of the air are concerned, the year may be divided 
into two parts—the winter period of five monthsfrom the end of the 
first week of November to the end of the first week in April, during 
which the average number of micro-organisms in 10 litres is 0.7, and 
the summer period of seven months, from the end of the first week 
in April to the end of the first week of November, during which the 
average is 40.1. The remarkable paucity of micro-organisms in the 
air during the five winter months is to be accounted for by the contin- 
uous frost and the repeated falls of snow, which prevent dust particles 


1 The ratios for November 10th and December Ist, 1908, and for March 25th, 
1909, could not be taken into account in obtaining this average, owing to their 
being indefinite. 
