538 Proceedings of the Royal Society of Edinburgh. [Sess. 
the clouds revealed the dusty condition of the air, which shows that the 
high numbers observed on this day were not entirely due to the sunshine 
effect but to real dust. Then, again, the limit of visibility was only 40 miles, 
which is decidedly low for north-westerly winds. 
On the 7th the numbers werfe very variable ; but as there was much 
sunshine, the high readings were probably of the abnormal sunshine kind. 
This is supported by the fact that the numbers, which had been over 5000 
during the day, fell to 375 in the evening, and also by the fact that the 
limit of visibility had increased to 70 miles. The observations made on 
the 8th were similar to those of the previous day, and the conditions were 
the same except that the wind had veered slightly. The numbers were low, 
336 per c.c., in the morning, but, under the influence of the sun in a cloud- 
less sky, soon began to increase, and rose to 6000, falling a good deal in the 
evening. The high numbers on this day were evidently due to sunshine, as 
the sky all day was cloudless. This conclusion is confirmed by the observa- 
tions on the haze, the limit of visibility having increased to 100 miles. 
Every effort was made on this occasion, as on the 6th, to check all observa- 
tions, sixteen different tests being made at different positions, at sea-level 
and at different heights up to 600 feet. 
On the 9th the conditions remained much the same. The sky was cloud- 
less, and the wind N.N.E. but stronger. No low numbers were observed on 
this day, neither morning nor evening, the readings being always over 3000 
and rising to 6000. This rise would be due to sunshine effect, but probably 
morning and evening readings were not greatly affected by sunshine, as 
the limit of visibility had decreased from 100 to 40 miles. On the 10th the 
conditions were but little changed. It was still cloudless, but the wind had 
fallen and become variable. The numbers were never low, and the air was 
of only medium clearness. On the morning of the 11th the sky was 
still cloudless, but the wind had changed to W. The humidity had greatly 
increased, and, as the numbers were still high, the air thickened and the 
limit of visibility was reduced to 15 miles. 
On looking at the figures for Appin during the thirteen days one naturally 
asks, Why did the numbers at the beginning of the period vary so much 
from those at the end ? Throughout the first half of the time the numbers 
were low, while in the last half they were high, though the wind was coming 
all the time from nearly the same northerly direction. An examination of 
the weather charts shows that during the first half the circulation over 
the place of observation was cyclonic, while during the last half it was 
anticyclonic, and that the nearer the centre of the anticyclone came 
to the place of observation the higher the numbers were. It has been 
