AIR CURRENTS IN RELATION TO AVIATION 359 



We have a very good example in the case of the convection 

 of dust from Africa, in 1903 (6), which is represented in fig. 8. 

 The charts for February 19, 20, 21 and 22 show the distribution 

 of pressure over Europe, North Africa and the Atlantic. The 

 other two charts show the lines along which air may be 

 inferred to have flowed during the four days from the north 

 of Africa on the one hand and from Canada on the other ; 

 the two streams were brought into parallelism over the British 

 Isles and the invasion of the territory of the warm current by 



Fig. 9. — Charts of isobars for I p.m., 2 p.m., and 3 p.m. on February 8, 1906, showing 

 the positions of the primary and secondary line-squalls at those hours. 



the air of the cold current explains the occurrence of the 

 rainfall which brought down with it the African dust. 



It appears to be the condition of things thus arranged 

 that gives rise to the phenomena of line-squalls, which we 

 may illustrate by the example of February 8, 1906 (7), repre- 

 sented by the diagrams (figs. 9, 10, 12). Similar diagrams 

 have been prepared for October 14-15, 1909(8). Fig. 9 shows 

 the distribution of pressure at three consecutive hours. The 

 dislocations of the isobars show the positions of the sudden 



