54 



ATMOSPHERIC DUST. 



Tahle XXXV. Mechanical Composition of Sliower Dust fallen at Rock Island, ill., 



February, 1896. 



T.pnjith 



of diameter 



ill mm. 



16—8 

 8-4 

 4—2 

 2—1 



1— ^ 



1 1 





1 1 



1 _ 



J 



64 

 1 _ 



TITS 





Prom snow in 

 timber. 



IS.? 



From snow just 



above tbft river 



blofls. 



From snow near 

 a ravine. 



From snow close 

 to a tree. 



tr. > 



3.2 



43.0 



4(j..j 



5.6 



^ 



Table XX XV 



(Continued). 



156 





From snnw on an 

 open field. 



Average. 



tr. • 



1.6 



30.9 



54.3 



11.4 



.4 



I 



4 

 42 

 44 



Over level areas, as out on the ice 

 of the river awav from the banks, 

 and on an open field, a g-reater 

 proportion of fine dnst is notice- 

 able (Xos. 146, 150 156). due 

 most likelv to a more even pro- 

 gression of the atmosphere per- 

 mitting more of its load to settle, 

 while near places where timber or 

 topographic contours liad set u]) 

 convection currents, less of the 

 .finer dust seems to have been 

 able to comedown (Xos. 144, 152, 

 153). In the averages from each 

 of these three showers the me- 



