38 



ATMOSPHERIC DUST. 



the atmo.spliere. is finer than volcanic dust, most of which 

 consists of isarticles ranging fi-om one eigth to one thirty- 

 second of a millimeter in diameter. 



Somewhat siniihir inferences may also be made fi-om 

 the natiii-p of some dust taken close to wagon roads, 

 where it was raised Ijy passing yehicles and sifted as it 

 fell by gentle winds. In some dust of this kind, whirh fp]] 



Table XXfll. Mechanical Composition of Dust coliected close to a travelled Wagon Road. 





9 



7 



98 



99 



100 





Length of 



diameter in 



mm. 



Dnst taken 5 ft. 

 from a road near 

 Baltimore, Md. 



Dost taken 15 ft. 

 from a road near 

 Baltimore, Md. 



Dnst taken 25 ft. 

 from a road near 

 Rock Island.nis. 



Dnst taken 25 ft. 

 from a road near 

 Baltimore, Md. 



Average. 



16-8 













8—4 













4—2 













2-1 



A 









.1' 



1-i 



4.0 





.2 • 







•7 1 



1.2 





h-i 



6.7 





.8 



tr. ' 



.7 



2.0 





i-i 



23.7 



1 



3.3 



1.4 ! 



3.0 



7.9 



1 



i-iV 



26.1 



1 



31.3 h 



29.2 ■ 



0.6 1 



24.0 



L 



T^-aV 



2.5.0 



1 



48.6 ■ 



4.-,.l fe 



■'"^.O ■ 



44.2 



■ 



A-sV 



9.2 



i 



14.8 1 



23.1 1 



21 7 r 



17.2 



r 



6'— Ti^ 



4.3 





.7 ■ 



.5 



4.2 r 



2.4 





1 3S 2 5 6 



.6 





.1 ' 





.6 1 



.3 





five feet from the road, twenty-three per cent was fine sand 

 and eleven per cent cwas omposed of still coarser grains 

 (Tab. XXIII I. In some other dust, which feU ten feet 

 farther away, tliere was only a little over four per cent of 

 the coarse grades. In this sample very fine sand forms 

 thirty-one per cent. Ten feet still farther out this grade is 

 rex:)resented by less than ten per cent. These grades evi- 

 dently easily settle out of gentle atmospheric currents. 



