THE MICROBIOLOGY OF THE ATMOSPHERE 



In Experiment i (28 July 1933), on a lawn near the Middle Neva 

 River, Elagin Island, Leningrad, approximately 1-2 X 10^ spores of 

 Tilletia caries were disseminated into the air through gauze, at a height 

 of about 80 to 120 cm. above the ground. According to anemometer 

 readings the wind varied from 0-5 to 4-0 metres per sec, but sometimes 

 fell to a complete calm; its direction was also variable. Two glass slides 

 were placed at each trapping position, the numbers of spores trapped 

 being shown in Table V. 



TABLE V 



RESULTS OF DISPERSAL OF SPORES OF Tilletia caries, 



Experiment i (Stepanov, 1935) 



Number of spores per cover-glass 18 X 18 mm. (average of 2) 



Angle 

 of slide 

 to wind 



-20° 

 — 10° 

 + 30° 

 + 45° 

 + 55° 

 + 65° 



+ 75° 

 + 85° 



At 5 metres 



from place of 



dispersal of 



spores 



204 



435 

 964 

 1 198 

 659 

 341 

 365 

 20 



At 10 metres 



from place of 



dispersal of 



spores 



23 



45 



212 



587 



123 



24 



5 



ID 



At 15 metres 



from place of 



dispersal of 



spores 



4 

 19 



207 



87 

 77 

 26 

 26 

 9 



At 20 metres 



from place of 



dispersal of 



spores 



49 

 142 



15 



7 

 53 

 14 



Experiment 2 (5 September 1933) was made at the same place as the 

 previous one. This time a mixture of spores of Tilletia caries and Bovista 

 plumbea was disseminated through a small sieve at a height of about 150 

 cm. Scattering of the spores occupied 15 minutes, after which 30 to 35 

 minutes were (perhaps unnecessarily) allowed to elapse for the deposi- 

 tion of the spores. During this period the wind mostly varied from 2-3 to 

 3-0 metres per sec, but was sometimes calm. As shown in Table VI, three 

 slides were placed at each trapping position. Approximately i-8 X 10" 

 spores of Tilletia were used but those of Bovista were unfortunately not 

 estimated. 



Stepanov's results led him to an empirical law of spore dispersal 

 which was expressed as : y = C + a/sx, where 'y' = the distance at 

 which the spores were trapped, x = the number of spores deposited per 

 unit area of trap surface, s = area of trap surface, and 'C and 'a' are 

 parameters dependent on the conditions of the experiment. The number 

 of spores deposited is thus regarded as varying inversely as the first power 

 of the distance from an origin of co-ordinates that is not coincident 

 with the source. 



It will be shown later that Stepanov's formula, which is the first- 

 fruits of the experimental approach to the problem, needs modification 



52 



