ATMOSI'IIKUIC DUST. 



3i) 



Table XXI]'. Mechanical Com- 

 position of Dust taken on the Ice 

 in the IVIinnesota River. 



Of particles which are less than one sixtv-lourtli of a 

 millimeter in diameter, there are only small (juantities, 

 presumabl.y because such particles tardil^^ settle even in 

 ordinary low winds. 



Some dust which was swept by the wind from the banks 

 and the bottom lands of the Minnesota river and lodged 

 on the ice in its channel close by, shows about the same 

 composition as the average of 

 the last samples (Tab. XXIV). 



The dust from the railroad 

 coaches, the volcanic dust, the 

 dust from the wagon roads, and 

 this last sample from the ice of 

 the Minnesota rivei- may be said 

 to indicate that particles, which 

 are capable of suspension in 

 strong winds, must have a diam- 

 eter less than one sixteenth of a 

 millimeter in length, and that 

 particles with a diameter of less 

 than one fourth of this length 

 are hindered from promptly sett- 

 ling out of such winds. The lat- 

 ter pai't of this statement must 

 however be made with a limitation as to the quantity of 

 the load which is carried. Should this be increased be- 

 yond a certain limit, flocculation will take place, and then 

 even finer dust will soon be brought down. 



Fifty-six samples of dust capable of prolonged sus])en- 

 sion in the atmosphere have been studied, and will here 

 be described under three divisions: 1) dust collected 



Length 



ol diameter 

 io mm. 



101 



16-8 





8-4 





4-2 





2—1 





l-l 



tr. 



l-i 



.6 , 



i-i 



6.1 i 



i-iV 



16.6 1 



iV-sV 



53.4 ■ 



1" 1 

 3^ ur 



20.7 1 



1 1 



Oi 128 



1.0 



TTS ^T¥ 





