THE GEOLOGY OF GXBKALTAK. 



537 



poraneous with the existence of land-passages between Europe and 

 Africa, and then let us inquire what effect such a sea under these 

 conditions would have upon the climate of Europe. 



At present the prevalent winds in the Mediterranean are northerly, 

 and this is due in chief measure to the influence of the desert. The 

 superheated sands exercise, as M. Martins remarks, a thermometric 

 aspiration from the north towards the south. Even in winter, al- 

 though the winds of the Mediterranean are somewhat variable, yet 

 northerly currents still predominate, for the simple reason that the 

 atmospheric pressure is lower in Africa than in Central and South- 

 ern Europe. In the Straits of Gibraltar the prevalent winds are 

 westerly and easterly. This is seen in the following Table, given by 

 Prof. Coffin in his elaborate treatise in the ' Smithsonian Contribu- 

 tions to Science' * : — 





Relative prevalence of winds from the 







Time of the 



different 



points of the compass. 





Direction of 



S a 



CD c*_i 





d 



IN 

















year. 



Eh 

 O 





§ 





1 







°J 



o 



J 



'eg 



Resultant. 



O t« 

 VS o 

 o3 -+J 





6 



fc 



H 



m 



zc 



GQ 



fc 



O 





P3 



January... 



1 



2 



7 



4 



2 



3 



5 



7 









February 



1 







6 



2 



1 



5 



4 



9 









March . . . 



1 



1 



6 



2 



2 



8 



3 



8 









April 







1 



3 



4 



1 



6 



3 



12 









May 











5 



4 



1 



8 



3 



10 









June 







1 



8 



5 



1 



8 



3 



4 









July ...... 







1 



11 



4 



2 



3 



5 



5 









August ... 



1 



1 



10 



4 



1 



4 



4 



6 









September 



1 



1 



9 



3 







3 



5 



8 









October . . . 



1 



2 



9 



3 



1 



6 



3 



6 









November 



1 



2 



9 



3 



1 



3 



4 



7 









December 



1 



2 



6 



3 



2 



3 



1 



13 









Spring . . . 



1 



2 



14 



10 



4 



22 



9 



30 





S. 82° 39' W. 



•26 



Summer... 



1 



3 



29 



13 



4 



15 



12 



15 





S. 81°55'E. 



•11 



Autumn... 



a 



5 



27 



9 



2 



12 



12 



21 





N.20° 19' E. 



•05 



Winter ... 



3 



4 



19 



9 



5 



11 



10 



29 





N.54° 28' W. 



•14 



The Year 



8 



14 



89 



41 



15 



60 



43 



95 





S. 86° 59' W. 



•07 



It is pretty clear, then, that the present climate of Gibraltar is 

 affected to only a small extent by the presence of the great Sahara 

 desert to the south. The only winds experienced at the Rock which 

 seem to have come from the heated sands of Northern Africa are 

 those that blow from a south-easterly quarter. This south-east 

 wind, or " Levanter," is a variety of the Sirocco. It is hot and 

 often loaded with impalpable dust ; at Gibraltar, while it blows, a 



* Vol. xx. p. 439, 



