Dust W Juris. 321 



• one mile, that the column was approximately vertical, and that 

 the angle of elevation was 80°, the dust whirl would be not far 

 short of six miles in height; and if it were only half a mile awav 

 (which, however, seemed altogether too short a distance), tlie 

 height would approximate towa^rds three miles. 



These figures, are probably far too high, due, perhaps, to the 



.assumption that tlie dust column was approximately vertical. 

 The column might appear to be vertical, and yet could perhaps be 

 much bent towards the observer. This would materially reduce 

 the figures. 



Another means of checking the height is to ascertain the height of 



•either of the two clouds associated with the dust columns, but no 

 data have been obtained fpr this. These clouds were two of many 

 similar scattered over, all ai:)parently of moderate height. Again, 

 if the general average height that these clouds form at Avere known, 

 an idea of the height of the dust whirl would be obtained, but 

 there appears to be hardly any information available as to such 

 ■clouds in Austi-alia in this connection. Records of other countries 



.show that the upper surfaces of cumulus clouds may be over 3000 

 feet high. 16 There can be no doubt, howevei'. that this particular 

 <lust whirl was of great height, reaching probably to several thou- 

 sands of feet above the earth's surface, and the quantity of dust 

 raised must have been enormous. When first viewed close to tlie 



.sun's rays, the dust whirls presented a majestic spectacle. 



The writer is indebted to Dr. Griffith Taylor for a reference to 

 Mr. Quayle's ISIemoir on Clouds, and for some information con-. 



'Cerning clouds, whicli lie kindly obtained from Mr. (^)uayle. 



Diifif Whirls i/i Felaf/oii to Erosion. 

 Dust whirls must play an important part in the erosion of sub- 



.arid Western Australia. From the prece<liug table it will be 

 recognised that they are fairly numerous, even in a small area, 

 and that they include columns of dust of great height. It must 

 also be remembered that, unless high, numerous whirls, even com- 

 paratively close to an observer, are not seen hy liini. If the whole 



■ of the sub-arid portions^ of the State be considei'ed, a vast amount 

 of fine sand, and dust must be displaced even in the course of one 

 favourable day. The m^iterial is either lifted well into the air, (u- 

 is dragged along or kept close tx) the surface of the ground. In 



-.the former case the material is chiefly fine dust, and in the latter 



16. Davis. Op. cit., pp. 179 and ISO. 



