MECHANICAL WORK 13 



more considerable. The dustiness of the atmosphere in dry regions 

 during wind-storms is familiar proof of the efficiency of the wind as 

 a carrier of dust. 



Under special circumstances, it is possible to determine roughly 

 the distance and height to which dust is carried. In the great 

 eruption of Krakatoa in 1883, large quantities of volcanic dust 

 (pulverized lava) were shot up to great heights into the atmosphere. 

 The coarser particles soon settled; but many of the finer ones, caught 

 by the currents of the upper air, were carried around the earth in 

 15 days, and some of it traveled round the earth repeatedly. Its 

 presence in the air was known by the historic red sunsets which it 

 caused. 1 



Dust from volcanoes is shot into the atmosphere rather than 

 picked up by it. Dust picked up by the wind is perhaps transported 

 as widely, but, after settling, its point of origin is less readily deter- 

 mined. It would perhaps be an exaggeration to say that every 



Fig. 5. Vertical face of loess near Huang-tu-Chai in northern Shan-si. The 

 vertical faces are the result of erosion. (Willis, Carnegie Institution.) 



square mile of land has particles of dust blown from every other 

 square mile of dry land; but such a statement probably would in- 

 volve much less exaggeration than might at first be supposed. 



1 A brief account of the influence of the dust on sunsets is found in Davis's 

 Elementary Meteorology, pp. 85 and 119. 



