660 POPULAR SCIENCE MONTHLY. 



experiments, it appears that two ounces of dust suspended in 

 about four thousand cubic feet of air render it as thick as it must 

 have been, at the least, in the storms described. This would 

 make about two thousand tons to a cubic mile. It should be 

 added that these experiments were made in a wind moving about 

 eight miles an hour, and with dust quite fine enough to be sus- 

 pended in such a wind. A considerably larger quantity would no 

 doubt be required if the material were to be coarser, such as 

 would be carried by a strong wind. The estimate is therefore 

 believed to represent a minimum for such storms as are described 

 above. 



Another perhaps less reliable estimate may be made from ac- 

 counts which describe the drifting sand, thus : " The sand drifts, 

 as snow does, and has attained such a depth as to cause a fear 

 that vegetation in the simoom's path will be greatly damaged." 

 " Drifts of sand one foot high were piled up in thirty minutes 

 on a railroad track/' " Cuts [along a railroad] were filled with 

 immense drifts, which averaged about two thirds sand and one 

 third snow." "At Cheyenne Wells, Colorado, thirteen cars of 

 sand were taken from the depot platform" (after a storm). 

 " Tracks were obliterated [by drifting sand] and the whole land- 

 scape was changed." 



Under such conditions it may be surmised that a drift of 

 twenty-five tons of sand might be deposited during six hours 

 from a current of air forty feet wide from the lowest ten feet, in 

 the lee of some intercepting obstacle, as in a railroad cut. In 

 fact, such instances are on record. The velocity so near the 

 ground would not exceed fifty miles an hour. Twenty- five tons 

 may therefore be carried by 633,600,000 cubic feet of air, which 

 makes nearly six thousand tons to a cubic mile. It is by no 

 means likely that all, or even the greater part, of the sand carried 

 by the lowest ten feet of the atmosphere can be left in the drift, 

 and the estimate may again be much too low. 



Still another approximation can be made by experimenting on 

 the effects of dust in the atmosphere on the respiratory mechan- 

 ism of the human body. Such effects are referred to in the fol- 

 lowing paragraphs: 



" The wind sweeps down from the deserts and brings with it 

 sand in such quantities as to almost make breathing impossible." 

 " The sand was blown in stifling clouds about them." " During 

 the sand storm the air is so full of dust that it feels as if it were 

 impossible to breathe." 



By some simple experiments it has been ascertained that less 

 than two grains of mineral dust suspended in a cubic foot of air 

 interferes with inhalation in a normal way. This would make 

 about twenty thousand tons of dust to the cubic mile. It is pos- 



