DEPOSITION PROCESSES 



at I • I metres per sec. the part of the surface behind the leading edge was 

 almost free from deposit, and efficiency reached about 50 per cent of the 

 expected value only on the rearmost zone. At 1-7 metres per sec. efficiency 

 was almost zero over the whole slide. With gravity neutral or negative, 

 efficiencies were also almost zero. 



(ii) Turbulent deposition. As the wind-speed was raised still further, the 

 efficiency of the horizontal slide recovered ; but deposition cannot have 

 been due to gravity sedimentation, because at 9-5 metres per sec. the 

 amount deposited was almost the same on the under-side (g_) as on the 

 upper-side (g^) of a horizontal slide, and also on the two sides of a vertical 

 slide held parallel with the wind (go). At this speed turbulent deposition 

 is seen in its almost pure condition. As indicated below, there is some 

 evidence that this deposition may result from turbulence generated by 

 the bluff edge of the slide itself. 



At 0° (horizontal slide) and higher wind-speeds, although deposition 

 was turbulent, gTaviry appeared to interact with the process in some way 

 that is at present obscure. With gravity neutral (go) and wind-speeds of 

 57 and 9-5 metres per sec, deposition was higher at the leading and trailing 

 edges than in the middle of the slide. With gravity negative (g_) at 9-5 

 metre per sec, however, the leading edge showed an anomaly, having a 

 deposit 8 to 10 times that found with gravity positive. With gravity 

 negative at 57 and 3-2 metres per sec, deposition behind the leading edge 

 was negligible. 



DEPOSITION OF Lycopodium spores on inclined plane surfaces 



Deposition on a horizontal microscope slide is best regarded as a 

 special case of deposition on an inclined plane with the presentation angle 

 0°. A number of possible angles ranging from 0°, through 90° (vertical 

 slide) to 180°, were tested in the turbulent wind-tunnel, and results are 

 also sho\Mi on Fig. 13. 



(i) Impaction. Deposition by impaction should be zero at 0°, but it 

 would be expected to occur to some extent at all other presentation angles, 

 as at these angles the surface subtends the oncoming air-stream. With the 

 slide vertical (90°), the gravity effect should be neutral, and at low wind- 

 speeds of 0-5 to I- 1 metres per sec. the slide would not be expected to 

 generate turbulence. Deposition under these conditions should be due to 

 impaction only. In the tests, as wind-speed was increased (90°, 17 to 

 9-5 metres per sec), deposition increased over the whole surface and was 

 more uniformly distributed; but even at the highest speed tested the 

 deposit at the margin exceeded that at the middle. In this respect im- 

 paction on a plane surface contrasts strikingly with that on cylinders, 

 where the centre of the trace is always denser than the edges. 



(ii) Edge drift. The effect of the bluff edge of the slide in 'shading' the 

 leading edge has been referred to above. Behind the edge shadow, a 

 region of greater deposition caused by an edge drift might be expected. 



69 



