40 



ATMOSPHEIUC UL'ST; 



directly from the atmosphere by means of some appara- 

 tus: 2) dust which has settled out of the atmosphere on 

 surfaces more or less elevated above the gTound, as from 

 leaves of trees and from house-roofs, and 3) dust which 

 has settled out of the atmosphere on snow, on ice, or on 

 other surfaces nearlv on a level with the oround. 



DUST COLLECTED DIRECTLY FROM THE ATMOSPHERE. 



One of the devices used in collecting; dust directly from 

 the atmosphere consisted of some whisks of broom-corn, 

 smeared with g-lycerine. and suspended from a pole ninety 

 feet above the ground. The observations were made on 

 a bluff overlooking the ^Missis.sippi river at Eock Island 



Table XXr. Mechanical Composition of Dust collected directly from Ihe Atmosptiere by 

 means of Whisks of Broom- corn smeared with Glycerine, March, 1895. ) 



Length of 



dlaiueter in 



mm. 



Maximum Maxjznam 



hourly Telocity. hourly velocity, 



32 miles. 17 miles. 



Maximum 



hourly velocity, 



22 miles. 



Maximum 



hourly velocity, 



28 miles. 



16-8 

 8—4 

 4—2 

 2—1 



1 1. 



■ 1 



16 



._1 

 Si 



1 _ 



1 6 



1 1_ 



T2^ 64 



1 _ 



_ 1 



_ 1 

 F56 



*) The method used iu making the analyses given in this table was somewhat 

 imperfect and the propoi-tions of particles ranging in size from a diam . of J- to yW 

 of a millimeter is too large. 



