"MACKEREL SKY" 369 



wind. Most of the time the individual transverse 

 bars of cloud travelled fairly fast with the wind. 

 In Plates LXXIX. and LXXX. I have had 

 a photograph, which was taken looking south 

 at 5.15 p.m., reproduced twice, once in the 

 ordinary way as a positive print, once as a nega- 

 tive. The former shows the clouds as they actually 

 appeared. They were drifting rapidly, and this per- 

 haps accounts for the fact that they are symmetrical. 

 The shadow of each roll of cloud is thrown on 

 the next cloud to the left. In the negative the 

 clouds appear dark, and the eye, following as it 

 is compelled to do the high lights, sees a different 

 pattern. This is the pattern of the blue sky, which 

 by this device stands revealed in its true form. 

 The blue sky is thus seen to be standing up in 

 sharp-crested ridges, which fit in between, and keep 

 apart, the whirling air, filled with particles of mist 

 which constitute the clouds. Thus the blue sky, 

 not the cloud, is the true aerial ripple-mark, the 

 clouds corresponding to the eddies of air or water. 

 But in order that the true aerial ripple-mark should 

 be manifest, it is necessary to adopt my device of 

 reversing the light and dark parts. The junctions 

 of the ridges are particularly deserving of notice 

 as showing the completeness of the similarity 

 between the ripple-marks of sky and sand. 



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