ATM08PHEKIC DUST. 59 



the medium dust. This in part accounts for the low 

 perceiitag'es of the maxima, wliieh never exceed sixty per 

 cent of the entire weight of each sample and which range 

 down to thirty. The deci'ease from the maximum to 

 either extreme ingi-edient is uninterrupted, except in two 

 samples collected from rain-water, which eame from the 

 roof of a house. In these the coarse sand is present in 

 greater quantity than the medium sand. The slope from 

 the maximum toward the coarse admixtures is more 

 gradual than that toward the fine admixtures in nineteen 

 of the samples. Most of these were taken near the surface 

 of the ground in places favorably situated for the admix- 

 ture of local material, as from the trunks of trees, from 

 house-roofs, from ice near river banks, and from snow 

 near bare patches of ground. In thirteen samples the 

 decrease is, on the other hand, rather more gradual to- 

 ward the fine admixtures. Such is nearly always the case 

 when the dust has settled in slack wind, as in the hollow 

 cylinder, among the trees, or under shelter. In twenty- 

 five samples the two slopes are about equally steep. 

 These include most of the dust caught on surfaces 

 smeared with glycerine and some of the shower dust. In 

 an average of all the samples, owing to the large admix- 

 ture of local coarse materials in a few instances, the slope 

 is more gradual in the direction of these admixtures. 

 But the difference is slight (Tab. XXXVIII). 



The significance of this last feature is quite evident. 

 The elimination of the sand from the settling dust follows 

 the same law as the separation of material which is still 

 flnwr, from this dust. The greater vertical components in 

 the wind near the surface of the ground are able to keep 



