50 ATMOSPHERIC DUST. 



water from a honse-rooL Such rough surfaces as these 

 give a secure lodgement to g;rains of sand as well as dust. 

 From the analyses it is quite evident that some coarse 

 matei'ial is moved even by the gentle winds of the Missis- 

 si]:)pi valley. It may be that many of these grains are 

 raised by the aid of lighter objects to which they adhere, 

 such as bits of straw and leaves. But their abundance in 

 these last samples is best accounted for by the action of 

 occasional strong convection currents and by the in- 

 creased chances for larger grains to find lodgement on 

 rough surfaces. This may be infei*red from two analyses, 

 one of which gives the composition of some dust collected 

 from the trunk of a small tree by striking it repeatedly 

 with a hammer (Xo. 127), while the other shows the 

 ingredients in the material which remained on the bark 

 after this procedure and which was secured afterward By 

 washing (Xo. 132). The former has a small and the 

 latter a large proportion of the admixtures on either side 

 of the maximum ingredient. Aside from the greater pro- 

 portions of the extreme grades, which may be accounted 

 for by the diminished proportionate chances of the grains 

 of the maximum ingredient to find and maintain a secure 

 lodgement, all of these samples resemble those collected 

 on surfaces rendered adhesive by the application of glycer- 

 ine. The averages of these two series of samples corres- 

 pond closely for each grade. Both are perhaps, on the 

 whole, slightly finer than the dust which is constantly 

 floating in the air over the central part of the upper val- 

 ley of the Mississippi. 



