DUST 



18SS 



DUST 



flying dust is a physiographic agent that per- 

 forms a wonderful amount of work. Flying 

 dust, snatched from the deserts of Central 

 Asia, is borne across high mountains and has 

 filled the valley of the Hoang River, "China's 

 sorrow," hundreds of feet deep. The wind- 

 blown dust loess, it is called is brought into 

 the valley more rapidly than the river can 

 carry it away. As a result, the sediment which 

 the river tries to carry, but cannot, causes 

 overflows and floods that are terribly destruc- 

 tive. Years ago the Hoang flowed into the 

 delta of the Yang-tse-kiang ; now its mouth 

 is in the Pechili, several hundred miles north 

 of the Yang-tse delta. Flying dust filled and 

 leveled the valleys between the block moun- 

 tains of the Western highland region; it cut 

 the Gulf of California in two, leaving the 

 remnant now known as Imperial Valley; and 

 it was the chief factor in leveling the surface 

 of the plains between the Missouri River and 

 the Rocky Mountains. Wind-blown sand is 

 forever rearranging a considerable part of the 

 coast line of the United States, thereby mak- 

 ing the work of the Coast Survey a never-end- 

 ing task. 



The wave of sand which swept Cape Hen- 

 lopen, on the east coast of Delaware, is aji 

 illustration of the movement of flying and 

 rolling dust. It must be borne in mind that 

 any slight increase in the velocity of the 

 wind adds enormously to its carrying power; 

 doubling its velocity increases the carrying 

 power sixty-four-fold. The burning of the 

 shrubbery and trees left a denuded area of 

 sea sand. The wind immediately began to 

 blow the sand, which had been protected from 

 it by the shrubbery, and in a few years a 

 drifting dune about a mile long and seventy 

 feet high began to travel westward. In the 

 course of fifty years the dune traveled west- 

 ward a mile or more, stopping only when the 

 force of the on-shore winds was balanced by 

 that of the currents blowing from the land. 



In the foregoing cases it should be noted 

 that the tremendous effects of wind-blown 

 dust are confined to areas that have but little 

 rainfall and, therefore, little or no shrubbery 

 and vegetation. The flying dust in such re- 

 gions consists almost wholly of earthy matter, 

 and its composition does not differ from that 

 of the rock from which it is worn. 



Varieties of Wind-Blown Dust. In regions 

 of generous rainfall the ground is protected by 

 grass and other vegetation. About the only 

 denuded surfaces, sea and lake shores ex- 



cepted, are public streets, playgrounds and 

 areas in which constant treading prevents the 

 growth of vegetation. As compared with arid 

 regions the amount of flying dust is infini- 

 tesimal. Small as the quantity is, however, 

 flying dust, considered from the standpoint of 

 public health, is an extremely important fac- 

 tor of the air, especially in large cities. 



In order to carry on the study of flying 

 dust, microscope slides may be exposed on sills 

 about twelve feet from the ground. In the 

 following experiments they were exposed from 

 the sills of laboratory windows, on one side 

 facing several hundred feet of lawn, on the 

 other facing a much-traveled public highway. 

 The slides on the sill facing the lawn faced 

 also the prevailing northwesterly winds. The 

 flying dust caught on these slides consisted 

 mainly of soot from chimney stacks several 

 miles away, the pollen of flowers, the scales 

 of lepidopterous (scaly-winged) insects, the 

 eggs of insects, the smut and similar parasitic 

 growths on vegetation, meteoric iron, ash, 

 earthy matter and various particles of un- 

 known character. Slides exposed in rural sec- 

 tions of the country (all in Southern New- 

 York) gathered about the same kind of ma- 

 terial, containing perhaps a greater variety of 

 pollen, but less of chimney products. In re- 

 gions where soft coal is extensively used, soot 

 and other chimney products are the prevailing 

 dust content of the atmosphere. 



Flying Dust May Carry Disease. At all 

 exposures facing public streets the earthy mat- 

 ter composing the surface of the street top 

 dressing, together with horse manure, consti- 

 tutes the bulk of the flying dust. In several 

 instances, after the streets had been covered 

 with tar, the daily catch of dust was reduced 

 about ninety-five per cent, but the remaining 

 catch consisted almost wholly of manure. Be- 

 cause of its lightness this material is easily 

 carried by the wind more easily, indeed, than 

 earthy matter. It pervades the innermost 

 recesses of dwellings especially drawers and 

 closets of rooms that face dusty streets. Wher- 

 ever there are air currents they will carry dust 

 of this character. Practically all food sup- 

 plies, except those hermetically sealed, are 

 plentifully besprinkled with it. Fortunately, 

 by the time it has been rain-soaked, dried 

 and ground into fineness, it is comparatively 

 harmless more so, in all probability, than the 

 garbage, putrescent matter and sputum that, 

 thrown upon street pavements, sooner or later 

 find their way into the air as flying dust. 





