46 



ATMOSPHERIC DUST. 



Table XXIX. Mechanical Com- 

 position of Dusi collected directly 

 from the Atmosphere by Means of 

 Slack Wind in a hollow Cylinder, 

 Feb. 19, 1896. 



stances referred to. On the nineteenth of February in 

 1896, when there was a high wind and much dust in the 

 atmosphere over the Mississippi valley, one more sample 

 was taken in the cylinder, this time suspended onl_y ten 

 feet above the ground (Tab. XXIX). 



All of the dust caught in the cylinder, excepting two 

 samples, is coarser than that which was caught on adhe- 

 sive surfaces. The maximum 

 grade consists of coarse dust in 

 the former, while in the latter it 

 is medium dust. It appears that 

 the slack wind was not retained 

 in the cylinder long enough to 

 allow the fine particles to settle. 

 In this way the maximum has 

 been transferred toward the 

 coarse grades. If then, as we may 

 suppose, the dust carried by the 

 air was of the same average com- 

 position in both instances, the 

 rate of decrease from grade to 

 grade on either side of the maxi- 

 mum ought to be more nearly 

 equal in the dust caught in slack 

 wind. Such is also the case, as 

 may be seen from the averages of all the analyses of 

 each kind (Tab. XXX). It is quite probable also that 

 some of the coarse grains were shaken off from the adhe- 

 sive surfaces. An average of these two averages may be 

 taken as representing the nearest approximation to the 

 composition of dust ca,rried in the atmosphere at the 





126 



Length of 



diameter in 



mm. 



Maximum hourly 



velocity of wind, 



29 miles. 



16-8 





8-4 





4—2 





2—1 





1-i 





1 1 



■2" 4 





i-i 



tr. ■ 



1 1 



S IB 



2.8 



iF 33 



26.5 JL 



A-ttV 



55.2 ■ 



A 1 s s 



13.2 y 



Ti-s~'5T(r 



1.2 



