40 MK JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



impure. In Scotland, the southerly winds seem to be the most impure, while the 

 north-westerly ones are the purest. It would appear that dusty impurities can be carried 

 great distances, as they are found in air that has travelled over wide tracts of country 

 in which no pollution has been added. 



In studying the distribution of pressure for the periods of these observations in Scot- 

 land, it became evident that there was a certain relation between the isobars and the dust. 

 In all the cases wherever the isobars were situated east and west, the air was pure, and 

 the closer the isobars were the purer it was, which means that under these conditions 

 we have westerly winds, and as it comes from the Atlantic, the air is purer than any 

 other in our area. Further, if the isobars were situated north and south, even though 

 they were fairly close, the wind never brought very pure air, which means that under 

 these conditions we have southerly winds, and coming from the polluted districts of 

 our country and the Continent it brings much dust with it. It should be remarked that 

 during all the tests the relative position of the areas of high and low pressure was 

 such as to give us W. winds with east and west isobars, and S. winds when situated 

 north and south. 



Air seems to carry its impurities long distances. For instance, southerly winds at 

 Alford brought a good deal of dust though they had to travel over a considerable extent 

 of mountainous and uninhabited country before coming to the place of observation. As 

 bearing on this point, it may be remembered that the discussion of the Kingairloch obser- 

 vations showed, that when a cyclone had advanced over our area from the west, and had 

 given rise to a circulation of air from France over Holland and northwards over the North 

 Sea, that after the centre of the cyclone had passed, and we got northerly winds, the air 

 was not always under these circumstances pure ; though northerly winds under most con- 

 ditions are pure. It looks as if, under the conditions above described, the air from the 

 Continent had circled northwards, and passed over the sea to the north of the station, 

 where its curving movement has brought it as a northerly wind to the point of 

 observation. 



From these remarks it would appear that we may be able to get an idea of the 

 amount of dust in the air by studying the isobars at and before the date. When 

 the isobars are close, and situated in an easterly and westerly direction with the 

 area of high pressure to the south of the depression, the air will be pure. But if the 

 isobars are situated north and south, with the area of high pressure to the right, the air 

 will not be so pure. Again, if the isobars are wide the winds will be light, and the air 

 tending to become impure ; or, if the isobars are irregular, the circulation will be confusec], 

 and the impurity may be very great at times. Further, though N. winds in Scotland 

 are generally pure, as they blow from uncontaminated areas, yet it seems probable that 

 if they follow a period during which the air of the Continent has been circling northwards, 

 they may be sometimes impure. 



