30 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



it may really be southern air we are testing. This seems to offer a possible explanation 

 of the abnormal readings referred to, as the middle series of arrows in our diagram shows 

 that the circulation over the British Isles was confused on the 1st, 2nd, 3rd, 4th, 7th, 

 8th, 11th, 15th, 16th, 17th, 18th, and 26th. It will be noticed that these dates cover 

 most of the days on which the abnormal readings were got. When the general air 

 circulation is in a confused condition, we cannot expect the same uniformity in the air as 

 when it is regular. As might be expected, abnormal readings were not got on all days, 

 nor during the whole of the days, on which the circulation was mixed. It may, however, 

 be mentioned that the observations on Ben Nevis support this explanation, as on 1st, 

 2nd, 3rd, 4th, 6th, 8th, 11th, 25th, and 26th, the numbers were high at the Observa- 

 tory at some time of the day. With one exception, these are all days on which the 

 circulation was irregular. 



There is, however, another explanation possible. An examination of the weather 

 charts shows that all these abnormal readings were got after a certain distribution of 

 pressure and circulation. Whenever a low-pressure area appeared over our islands, and if 

 its centre passed to the S. of this station when it had moved to a position to the 

 S.E. or E. of Ben Nevis, the numbers went high with northerly winds during some 

 period of the day. When a cyclone moves along this route, the effect is to give rise, 

 while it is approaching our area, to south-westerly winds over France and Belgium. 

 This drives the impure continental air to the N.E. Then as the centre of depres- 

 sion advances, this air is driven northwards ; and when the centre of the cyclone lies to 

 the E. of this station, the air which moved northwards curves round and arrives at our 

 station from a northerly direction. By this explanation, the impurity of these northerly 

 winds was not due to contamination acquired in our area, but was due to impure con- 

 tinental air which had been driven northwards over the North Sea, and had curved round 

 and come to the station as a northerly wind. This explanation would account for the 

 high readings got on the 6th, when the general circulation was regular, and the wind 

 was from the N.W. 



The irregular circulation within our own area, or the circulation northwards of con- 

 tinental air, may or may not be the cause of these abnormal readings. These explana- 

 tions are offered at present for want of better, though one or other, or both, may possibly 

 be true ; yet the evidence is far from conclusive. It would be difficult, by studying 

 the weather charts, to trace the different masses of air on these days from the place of 

 observation to their sources. 



Dust and Wind at Kingairloch. 



The effect of the direction of the wind is very evident in these Kingairloch observa- 

 tions. It should be mentioned that winds from S. to E. at this station bring the most 

 polluted air, being from the most densely inhabited district, while winds from S.W. 

 to N. blow from the least inhabited areas. From the table and the diagram it will be 

 seen that from the afternoon of the 2nd to the afternoon of the 10th, with the exception 



