ATMOSPHERE OF GREAT BRITAIN AND ON THE CONTINENT. 643 



On 1 4 days the numbers rose abnormally high : 



on 9 of these the readings were taken in anticyclonic areas, 

 and on 4 days the readings were taken in cyclonic areas. 



From the above it will be seen that there is a decided tendency for the readings to 

 remain low in cyclonic areas and to be abnormally high in anticyclonic areas. But, 

 unfortunately, this brings us no nearer the explanation of the phenomenon, as these are 

 the conditions which bring us more or less clouded skies, which we already know are 

 connected with the variations under discussion. 



At present, amidst so much darkness on this point, one is apt to suggest impossible or 

 at least improbable explanations. As no satisfactory conclusion has been arrived at, 

 anything said here must be more or less of the nature of a guess in the dark. When I 

 wrote Part II. I offered the following explanation : — All these abnormal readings come to 

 us after a certain distribution of pressure and air circulation. When a low-pressure area 

 appeared near our islands and passed to the south of Scotland, after its centre lay to the 

 east of our station, high numbers were obtained. W T hen a cyclone passes along that 

 route it drives the impure Continental air northwards over the North Sea ; and when the 

 centre of the depression has arrived at a point to the east of Scotland the impure air will 

 have continued to curve round, and in this way come to Kingairloch as a N.W. 

 wind ; and it was suggested that the high numbers might be due to impure air circling 

 round and coming with a N.W. wind. We might explain the sunshine always 

 accompanying the high numbers by supposing that the cloudless air had started with 

 less moisture in it — it was therefore neither cleansed by rain nor clouded when it came 

 to our station. From the information we now possess, this explanation must be abandoned, 

 because though it might explain why the high dust should always be accompanied by 

 sunshine, yet it does not explain why the high dust should only come during the clay and 

 the amount always fall with the setting sun. 



If we examine the Ben Nevis observations we do not find that abnormal readings were 

 got on all the days when they were obtained at Kingairloch, though they were observed 

 on some of the days, and it will be noticed that they were obtained either when the air 

 circulation over our area was mixed or when there was little wind on the Ben. Of 

 course the conditions on the Ben are different from those at Kingairloch, and while we are 

 in sunshine they are often in cloud. On trying to work out the effect of sunshine on 

 Ben Nevis, to see if there was an increase in dust at high level when, there was one at low 

 when the conditions were the same at both stations, I find the Ben Nevis observations 

 on these days provokingly defective, either from absence of observations, or more generally 

 from observations marked "Doubtful." Under these circumstances, this point cannot be 

 pursued till more complete observations are obtained. 



Another possible explanation of these very high readings is that they are purely local. 

 But here again the fact that their appearance depends on sunshine seems at once to 

 disprove it. It was, however, thought advisable to test this point by an examination of 



