686 J. A. IDDEX COMPOSITIOX OF CLASTIC SEDIMENTS 



Dust collected by apparatus from the atmosphere. — One of the devices 

 the Trriter used in collecting dust directly from the atmosphere consisted 

 of some whisks of broom-com smeared with glyceriae and suspended 

 from a pole 90 feet above the ground. The collecting was made on a 

 bluff overlooking the ^lississippi Eiver at Eock Island, iu lUiaois. The 

 whisks were taken down once a day and washed in water, which was al- 

 lowed to stand until the dust had settled. This was then removed, dried, 

 and ignited. One series of such samples was secured during the month 

 of March, in 1895. These were mixed ioto five larger samples, each of 

 which represented days with maximum hourly wind velocities ranging 

 between fixed limits. The range of these velocities duriag the month 

 wa^ from 12 to 33 miles per hour, and the quantities of dust taken were 

 roughly proportionate to the sixth power of these velocities, ranging 

 from one-tenth of a gram to 50 grams. The analyses were imperfect in 

 the coarsest grades and have been omitted. They do not indicate that 

 there was any increase in the size of the particles transported during the 

 days having the strongest wind, as might have been expected. The maxi- 

 mum in each of the samples occurs in the medium dust. 



In June of tlie same year material was collected in the same manner 

 and at the same place daily for one week, and a separate analysis was 

 made of each catch. The maximum hourly velocities of the wind for 

 each day ranged from 9 to 20 miles. In this case also there was a cor- 

 respondence between the wind velocities and the ■ quantities of the dust 

 caught. On examining the analyses, it is seen that the coarse admix- 

 tures rather decrease than increase with the speed of the wind. The fine 

 ingredients are quite as well represented for the days with high winds 

 as for days with low winds. 



Some dust was collected at the same place and at the same height by 

 suspending two pieces of muslin held horizontally on a frame. The mus- 

 lin was smeared with glycerine, to which the dust adhered. The catch 

 was secured by washing and allowed to settle as before. One sample 

 consisted of a mixture of daily catches taken during part of June and 

 part of July, in 1895. These were thoroughly mixed before the analy- 

 sis was made. Two samples which were taken, one on the upper cloth 

 and one on the lower, on the nineteenth of August the same year, were 

 separately examined, as was also some other material collected in the 

 same manner under some trees in a grove about a quarter of a mile from 

 the pole previously referred to. The dust taken in this way resembles 

 perfectly that which was caught on the broom-corn. 



Another device for collecting dust from the atmosphere consisted of 

 a hollow cylinder, with apertures on the side for receiving the wind and 



