THE MICROBIOLOGY OF THE ATMOSPHERE 



Tests were accordingly made in the Rothamsted wind-tunnel, with a 

 15-5 cm.-square constriction upwind to generate turbulence. Lyco podium 

 spores (32 ju- diameter) were blown in at a point on the axis 18 cm. 

 upwind of the constriction, and, after a diffusion path 63 cm. long, the 

 spore-cloud reached the trapping section where a stationary and a rotating 

 cylinder, each 0-5 cm. in diameter, were exposed simultaneously. 

 The rotating cylinder, which was placed vertically across the axis of the 

 tunnel, consisted of a steel rod mounted in a ball-race secured flush with 

 the floor of the tunnel at one end, and connected by a sleeve of stout 

 rubber tubing to the spindle of a 225-watt 'Universal' motor mounted on 

 the roof of the tunnel. The stationary cylinder was 2 cm. to one side and 

 2 cm. upwind of the rotatable cylinder, and both carried adhesive coatings 

 of cellulose film which, after exposure, were removed and scanned under 

 the microscope to measure the deposit. Speed of rotation of the cylinder 

 was controlled by a 'Variac' transformer and was measured by a strobo- 

 scopic lamp. The range of speeds that could be tested w as limited by the 

 steel rod which began to bend at speeds above 7,000 r.p.m. 



With a wind of i-i metres per sec, efficiency was substantially un- 

 changed until the peripheral speed of rotation attained approximately the 

 speed of the wind; at higher rates of rotation the efficiency decreased 

 rapidly, reaching zero before the peripheral speed of the cylinder reached 

 twice that of the wind. At a lower wind-speed of o-68 metre per sec, 

 efficiency of the cylinder, when rotating at 0-4 times the wind-speed, fell 

 to 53 per cent of the stationary cylinder, and to 24 per cent at 0-82 times 

 the wind-speed. These results suggest that the centrifugal effect is suffi- 

 cient to decrease impaction to zero. The phenomenon would be interesting 

 to explore at a wider range of cylinder diameters and wind-speeds (the 

 work of Brun et al. refers to much higher wind-speeds than these). 



Impaction on Plane Surfaces at Various Angles to Wind 



The interest in deposition on narrow horizontal and vertical strips 

 lies in the widespread use of microscope slides for routine trapping of 

 fungus spores and pollen. 



The theory of gravity deposition assumes that the air flowing past the 

 surface contains a large population of particles distributed at random. The 

 particles fall at their terminal velocity, v^ cm. per sec, and the wind 

 blows horizontally at u cm. per sec. A plane surface of area i sq. cm. faces 

 the wind, making an angle d with the horizontal plane, as shown in Fig. 

 12. Then only particles contained in the rectangular skew prism ABCD 

 EFGH, have trajectories in the free air which would carry them to the 

 surface during time t sees. The volume of this prism is given by 



V = t(u sin d + V, cos 9) cc. 

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