34 



ATMOSPHERIC DUST. 



Tahle XXI. Mechanical Compo- 

 sition of some Dust taken from a 

 Window Sill in a House in Yuma, 

 Arizona. 



Length of 



diameter In 



mm. 



89 



had as much as seventeen per cent of fine ^and, and one 

 had less than one jDer cent (Tab. XX). In five of these 

 samples the maximum occurs in the grade of veiy "fine 

 sand, Avhich is next in fineness to the maximum grade of 

 the dune sand ; in seven of the samples it occurs in the 

 coarse dust : and in one it is in the next finer grade. The 

 small percentages of the coarser grains is no doubt in 



part due to the reduced veloc- 

 ities of the currents entering the 

 coaches. Analogous causes maj 

 have affected the perfection of the 

 sorting in these samples, which 

 varies considerably, ninety parts 

 in a hundred being distributed 

 among four grades in some in- 

 stances and between only two in 

 some. But the differences in the 

 speed of the trains and the dif- 

 fei'ences in the mechanical compo- 

 sition of the surface deposits 

 along the railroads must also be 

 taken into account. Nor was the 

 sampling uniform. In some in- 

 stances the dust was taken after 

 heavy winds and in others during calm weather, some- 

 times it was gathered up from the windoAv sills and some- 

 times from the seats in the coaches. Some of it was 

 brushed from the wearing apparel of a passenger. Taking 

 all these modifying circumstances into due consideration 

 and remembering that the currents of wind which follow 

 a running railroad train are quite as powerful as the cur- 



16-8 





8-4 





4-2 





2-1 





1-i 



.3 



i-i 



.6 



i— ^ 



7.3 1 



1-iV 



46.5 ■ 



iV-=fe 



36.4 ■ 



1 1 



3 3 6 4 



7.0 



64 138 



.8 



128 3 56 



tr. '. 



