THE MICROBIOLOGY OF THE ATMOSPHERE 



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IMPACTOR 

 UNIT 



Automatic Volumetric 

 Spore Trop 



Fig. 17. — Diagram of the Hirst automatic volumetric suction-trap: i, eleva- 

 tion facing wind; 2, plan of section through orifice; 3, elevation in side view; 

 C, clockwork drive for raising slide; O, orifice facing wind, with slide in holder 

 moving up past orifice; V, to vacuum pump. (Wind vane and cap to protect orifice 

 from rain are not shown.) (Illustration by permission of J. M. Hirst.) 



as the second jet of the Cascade Impactor), behind which is placed a 

 sticky microscope shde moving at 2 mm. per hour. In the course of twenty- 

 four hours a trace is deposited in a band 48 mm. long. This can be scanned 

 under the microscope longitudinally to obtain a daily mean, or it can be 



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