666 



MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



The above tables show clearly the much greater purifying power of our northern 

 areas, the great purifying power of the cloudy and rainy weather on the west coast of 

 Scotland being very well marked. The purity of the air in the Eastern Highlands, as 

 represented by the Alford numbers, is no doubt due to the same cause as the purity of the 

 West Highland air. The tables show it is only slightly polluted in its passage from the 

 Atlantic across the mountains of Scotland. 



It will be observed that the relative purity of the air from the different areas is the same 

 in both tables, but the contrast is not so great in the mean numbers as in the lowest, 

 while the mean of the lowest numbers in Atlantic air, Table XXIII. , is -^ of the mean of 

 the lowest in Mediterranean air ; the mean of all the observations in Atlantic air, Table 

 XXIV., is A of the mean of the Mediterranean air. The general conclusion from the figures 

 in the last table, is that the mean purity of the air from the Atlantic is five times 

 greater than that from the Mediteranean ; the Highland air is three times purer, while 

 the air from the Alps is twice as pure. It may be well to note here that we are dealing 

 in these tables only with the observations made by myself, and that lower numbers 

 have been observed on Ben Nevis, in Atlantic air, than are entered in Table XXIII. 



Table XXIV. 



Year. 





Mean Numbers of Particles per c.c. 



observed 



in air coming from — 





° § 



3 g 



Mediterranean. 



° o 

 S3 '■§ 



-2 > 



2 f-i 



"• CO 



Alps. 



° § 

 ? > 



,P to 



Highlands. 



,■-> CO 



Atlantic. 





£5 





££ 





&■£. 





£^ 







O 





o 





O 





o 





1889. 



] 



1600 



39 



698 



8 



697 



8 



481 



1890. 



2 



767 



21 



1030 



8 



703 



65 



337 



1891 . 



7 



1865 



31 



575 



16 



401 



74 



297 



1892. 



7 



2002 



22 



1341 



21 



468 



53 



366 



1893. 



11 



1363 



9 



1402 



33 



605 



58 



347 



Mean . 



28 



1611 



122 



892- 



86 



552- 



258 



338 



Dust and Temperature. 



No attempt has as yet been made to work out from the figures in Tables L, II. and 

 III. the relation between the amount of dust and the temperature. When treating of 

 this subject in Part II., it was shown that there is probably a relation between them, but 

 the conditions are too complicated, and continuous observations on many points are 

 still wanting before anything satisfactory can be said under this head. 



