ATMOSIMlKinr nusT. 



47 



place where these ubservntions \vei'(^ niude. It iiiny be 

 collected in low winds as w^ell as high, and though it 

 appears to be slowly settling, its general presence indi- 

 cates that it is easily held in suspension. 



Table XXX. Average Mechanical Composition of Dust caught on Adhesive Surfaces 



and In Stack Wind. 



Average duat 

 caught on adhe- 

 sive surfaces. 



Average dust 



caught In slack 



wind. 



General 

 Average. 



DUST TAKEN ON NATURAL SURFACES ABOVE THE GROUND. 



Several analyses have been made of dust found adher- 

 ing to surfaces of objects more or less elevated above 

 the ground (Tab. XXXI). Eight such samples were 

 washed from the foliage of trees, on which appreciable 

 deposits of dust may always be observed. The maximum 

 a-rade in this material is medium dust, but the lesser 

 weight and the smaller size of the particles smaller than 

 this renders them less subject to dislodgement by the 

 wind and by occasional shaking and rubbing of the 



