52 



ATMOSPHERIf DUST. 



of the medium. This was collected during- and after a 

 heavy wind in the early part of June in 1895. 



Thirteen samples wei'e gathered from the surface of ice 

 and from snow at Eock Island in Illinois and these repi-e- 

 sent three different showers. One such shower occured in 

 the latter part of November in 1894 (Tab. XXXIII), one 

 in the latter part of January in 1895 (Tab. XXXIV), 



Table XXXJII. Mechanical Composition of Shower Dust fallen at Rock Island, Ills., 



November, 1894. 



Length of 



diameter in 



mm. 



Taken on the 



ice of a small 



pond. 



From tlie ice of 

 the Mississippi 

 near the ban^. 



Average. 



and one in February in 1896 (Tab. XXXV). The selec- 

 ti(jn of these samples was made with a view to find out 

 not only the average composition of the sediment from 

 each shower but also the range of variation in composi- 

 tion which might be due to changes in convection currents 

 in the atmosphere and to the admixture of local material. 

 Dust gathered on the ice close to land contains eompar- 



