AIR SAMPLING TECHNIQUE 



times at I- 1 and 075 metres per sec., and by about 5 times at 3-2 metres 

 per sec, whereas at 9-5 metres per sec. the efficiency is decreased by one 

 quarter. 



The use of the 7-6 X 2-5 cm. microscope sHde at angles intermediate 

 between 0° and 90° would increase trapping efficiency at the wind-speeds 

 normally experienced in the open air near the ground. Hyre (1950) 

 observed that a slide with a presentation angle of 45°, exposed in the 

 field, caught more sporangia of Pseudoperonospora cubensis than either 

 a vertical or a horizontal slide at wind-speeds of up to 2-4 metres per sec. — 

 a result easily explicable from wind-tunnel work (cf. Fig. 14). It ought to 

 be possible to orientate a slide at 5° to the oncoming wind which should 

 give an efficiency varying only between o-6 and 1-4 per cent over the 

 range of i to 10 metres per sec; or at 10°, when its efficiency would vary 

 between 1-5 and 4-5 per cent over the same range of wind-speeds. An 

 alternative approach would be to make the slide tilt, according to the 

 wind's force, over the range of angles 10° to 30° as the wind-speed decreases 

 from 10 to I metres per sec. — over which range it would have a nearly 

 constant efficiency of 4 per cent M'ith Lycopodium. 



The vertical 7-6 X 2-5 cm. slide, widely used for research in plant 

 patholog}'', is an impactor trap ; it suffers from the defects of being rela- 

 tively inefficient as an impactor at low wind-speeds and of being highly 

 sensitive to changes in wind-speed. To avoid wrong conclusions and to 

 be able to translate deposit into time-mean concentration, it is necessary 

 to know the wind-speed under which deposition took place and to make 

 the necessary correction. This may be illustrated by considering the 

 deposit received from a cloud containing 10,000 Lycopodium spores per 

 cubic metre, using wind-tunnel data for two wind-speeds. At i-o metres 

 per sec. impaction at 5 per cent efficiency would deposit about 20 spores 

 per sq. cm. per hour. With an increase in the wind-speed to 9-4 metres 

 per sec, the deposit would increase to about 9,000 spores per sq. cm. 

 per hour — a 450 times increase in deposit without change in the number 

 of spores per cubic metre of air. A further source of error is that at low 

 wind-speeds the efficiency is low, and as the catch is therefore small it 

 has to be multiplied by a large factor, and the error of estimation becomes 

 great. 



(iii). Aeroconiscopes^ used first by medical workers and later by plant 

 pathologists, are now mainly of historical interest. They seem first to 

 have been used by Salisbury (1866) in the Mississippi Valley, but were 

 developed more fully by Maddox (1870, 1871) and Airy (1874) in England, 

 and by Cunningham (1873) in India (Chapter I). 



Aeroconiscopes of the Maddox and Cunningham t}'pe (Fig. 2) 

 have been used in plant pathological investigations by Christoff (1934) 

 in Bulgaria, and by A. A. Shitikova-Russakova {see Stepanov, 1935) in 



* The word 'aeroscope' has been used with some ambiguity, referring either to 

 this type of instrument or to bubblers of the type described by Rettger (cf. p. 97). 



95 



