THE MICROBIOLOGY OF THE ATMOSPHERE 



the cylinder. With low wind-velocities and with particles smaller than 

 spores, Brownian diffusion may play a major role in deposition. 



Fig. 10. — Observed relation between E per cent and k = Vsu/^dg. Solid lines from 

 Gregory & Stedman (1953). Broken lines = values for spheres, strips and cylinders as 

 predicted by Langmuir & Blodgett (1949), for <p = 10^. (Reproduced by permission from 

 the Annals uf Applied Biology.) 



The general principles of impaction have been made clear by Sell 

 (193 1) in connection with dust filtration. From aerodynamical considera- 

 tions Sell postulated that the efficiency of impaction, E, is related to a 

 non-dimensional constant, which may be written in a form convenient 

 to us as 



k = VgU/idg, 



where: v^ == speed of fall of particle in still air; u = speed of wind; 

 d = diameter of cylinder, strip, etc.; and g = acceleration due to gravity. 

 Sell derived the relation between E and k by observing the trajectories 

 of uniform droplets of Indian ink on paper bisecting a vertical cylinder 

 in a small wind-tunnel. It is not clear from Sell's paper what range of 

 conditions he tested. 



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