(544 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



the results of any observations that were not made at the usual place. The only possible 

 source of local pollution in N.W. winds at Kingairloch is a shepherd's cottage about 

 three-quarters of a mile up the glen, down which the N.W. winds blow, and there are no 

 houses to the right or left for many miles. The next house in a north-westerly direction 

 is another cottage at a distance of 5 miles. On the 14th July 1893, one of the most 

 abnormal of the days, I went to the windward of the shepherd's cottage and took some 

 of the afternoon observations and found the numbers high there also. It will be seen 

 that these observations fit into the curve the same as if they had all been taken at 

 Kingairloch. Again, the readings taken on the 23rd July 1892 were taken 4 miles up 

 the glen, and, as will be seen, they were very high. There are, however, observations 

 made on two days which must not be lost sight of. One of these days was the 24th June 

 1893. Mid-day and afternoon observations were taken on that day at a distance of about 

 9 miles from Kingairloch, and it will be noticed that, though the conditions were favour- 

 able for high numbers, yet they did not rise. A possible explanation is that the 

 observations were made in a cyclonic area, which we have seen is not favourable for high 

 numbers. Again, on the 4th July 1892 the observations were made at a distance of 4 

 miles from Kingairloch in sunshine and N.W. wind, and the numbers are not abnormally 

 high. This exception, however, is not so difficult to understand, as I find it was a purely 

 local N.W. wind due to the high mountains. The general circulation at the time was 

 S.W. There thus remains only the observations taken on the 24th of June 1893 to 

 suggest that the abnormal readings may be due to some local cause. Great weight must 

 not, however, be put on this exceptional case, as it will be seen from the diagrams that 

 at Kingairloch also readings not much higher were obtained in N.W. winds and sunshine. 

 If these abnormal readings at Kingairloch are due to local pollution, it is evident they 

 are of a nature and from a source of which we at present have no knowledge. 



A third possible explanation of the abnormal readings is, that sunshine under certain 

 conditions produces some change in the constituents of our atmosphere which gives rise 

 to something which forms a nucleus in supersaturated air. In Part II. p. 41, in dis- 

 cussing the Alford observations, it is pointed out that there was a relation between the 

 curves of dust, temperature, and sunshine. The longer the hours of sunshine, the higher 

 the number of particles. But as at Alford the southerly winds brought clear skies and 

 dusty air, it was difficult to make out whether there was anything of the nature of cause 

 and effect or not. These latter observations, however, seem to suggest that the sun 

 may cause an increase in the nuclei under certain conditions. The fact that the air, 

 on the days on which these abnormally high readings were obtained, was not hazed in 

 proportion to the number of particles, suggests that the nuclei on these sunny days and 

 N.W. winds are of molecular dimensions, and it even seems possible they may not be 

 nuclei at all while the air is dry, and only form nuclei in saturated or supersaturated air. 

 This consideration rather helps to support the sunshine theory of the origin of these very 

 high numbers. 



It is unnecessary to say anything further about these abnormal readings. It is 



