ATMOSPHERE OF GREAT BRITAIN AND ON THE CONTINENT. 37 



Another depression appeared in our area on the 19th. On that day its centre lay to 

 the S.W. of Ireland. This cyclone moved in a northerly direction, with its centre just 

 touching our extreme western coast. As the high -pressure area remained firm in the E. 

 at this date, this second depression, like the first, gave rise to regular and somewhat close 

 isobars ; but as their direction was again N. and S., southerly winds prevailed over our 

 area, and continued to bring impure air to our station, as,, it will be seen, the number of 

 particles remained high. Our station did not get free from this distribution of pressure till 

 after the observations were stopped, and the amount of dust remained high to the end of 

 the observations. 



Callievar. 



As on the previous visit to Alford, an ascent of Callievar was made in 1890 also. 

 The morning of the 22nd being fine it was selected for the purpose. At low level the 

 number of particles was high, and the wind slight and from the S. On arriving at 

 the top of the mountain the view was in striking contrast to what it was the previous year, 

 though the weather on both occasions was much the same. On the first visit the Cairngorms 

 and Lochnagar were quite distinct, though seen through some haze ; and the number of 

 particles was 262 at mid-day, and rose to 475 two hours later. In contrast with this, on the 

 second visit the air was thick and densely hazed, only a slight outline of the Cairngorms 

 being occasionally detected, while Lochnagar was quite invisible during the whole time. 

 The number of particles was 710 at 12.30 p.m., and rose to 1575 two hours later — i.e., 

 there were about three times as many particles at the time of the second visit as there 

 were at the time of the first, and there was a correspondingly thicker atmosphere. ■ 



Not only was the air much thicker on the second visit, but it also seemed to vary in 

 clearness in different directions and at different times. It looked as if the air w T as of 

 very different constitution, i.e., came from different sources, at its different parts. When 

 the second test was made at 2 p.m., the number of particles had greatly increased from 

 what it was at first, and the air had also got thicker. It was also observed that the air 

 was still thickening. Tests were, therefore, made at intervals, and from the table it will be 

 seen that the number rose to 1575 at 2.30 p.m. A little before this hour it was noticed that 

 the air was thicker than it had been half an hour before. At 3 p.m. the air to the W. was 

 again clearing, while to the E. it had got much thicker, and the limit of visibility in that 

 direction was much reduced. On testing the air at 3 p.m. it was found to be much purer, 

 the number having fallen to 1050. 



These observations show that between 2 p.m. and 2.30 p.m. a mass of impure air was 

 approaching from the west. This impure air thickened the atmosphere to the west and 

 caused the number of particles to rise at the place of observation. This mass of impure air 

 drifted across the mountain and passed to the east, after which the air to the west cleared 

 and the numbers fell, but the impure air in its passage eastwards thickened the atmos- 

 phere in that direction. My reason for entering so fully into these Callievar observations is 

 that the conditions were such as to give an opportunity of testing the hazing effect of 



