AIR CURRENTS IN RELATION TO AVIATION 361 



which destroyed the Tay Bridge on December 28, 1879. Line- 

 squalls or phenomena closely analogous thereto apparently 

 occur in all parts of the world. 



I have already said that in Greek mythology the line-squall 



Fig. 12. — Isochronous lines showing the travel of the front of the line-squall from the 

 Hebrides to Central France. The region of thunderstorms in England is marked off 

 by a dotted line. 



had a place, being personified by the Harpies. Let me quote 

 what Ruskin says about the matter in The Queen of the Air : 



"... I must go on at once to the fable with which you 

 are all generally familiar — that of the Harpies. This is always 

 connected with that of Boreas or the north wind, because the 

 two sons of Boreas are enemies of the Harpies and drive 

 them away into frantic flight. The myth in its first literal 

 form means only the battle between the fair north wind and 

 the foul south one : the two Harpies, ' Storm-swift ' and 

 • Swift-foot,' are the sisters of the rainbow — that is to say, 

 they are the broken drifts of the showery south wind and the 

 clear north wind drives them back; but they quickly take a 



