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



falling together. And one now naturally asks, What can be the relation between the 

 amount of dust and the number of hours of sunshine ? So far as the observations go at 

 present the reason for the close relation observed at this station is, that the same winds 

 — viz., southerly ones — that brought the high dust also brought clear skies, and when 

 the wind went either E. or W., though it brought a purer air, it also brought cloudy skies, 

 and the lower temperature may in part have been due to the clouds. 



Turning now to the observations made in the end of February and beginning of 

 March of 1891, let us see what information they give on the effect of the dust on the 

 temperature. As will be seen from the table, the amount of dust in the air in the end of 

 February was excessive, and from other observations, which are not entered in the table, 

 I find that during most of the month the air was very full of dust. On turning 

 now to the Meteorological Report for February it will be found that the average weekly 

 temperature for the stations in Scotland was above the mean, and was frequently very 

 high for that month. The warm weather of February was certainly accompanied by a 

 very dusty atmosphere ; the dust, however, may or may not have been the cause of the 

 high temperature. Turning again to the table, we see that the amount of dust suddenly 

 fell in the beginning of March to an exceptionally low figure, and that it continued low 

 for some days. On now consulting the Meteorological Eeport for March, we find that 

 the mean temperature for these days was not low, but over our area it was 2° above 

 the average. Here the relation between the dust and the temperature seems to break 

 down, as the air was very pure, and yet the mean temperature was high. Let us, 

 however, examine the conditions more closely, and I think it will be admitted that the 

 facts are open to quite a different interpretation. It is true the mean temperature was 

 above the average on the days when the dust was very low. But during all these days 

 the wind blew with great force and the skies were clouded, so that the temperature of 

 the air would be entirely governed by what the winds brought us. As it was winter, it 

 was probable that the winds would bring a temperature higher than the mean, just as in 

 summer they will probably bring a lower. If we are to find any effect from the low 

 dust observed during the first days of March, we must look for it in the conduct of the 

 air after the wind has fallen. The weather charts for March, from the 1st to the 

 6th, both included, all show the isobars to be close, regular, and across the map. On 

 the 7th the isobars began to widen out, and from the 8th on to the 15th they were wide 

 and irregular, indicating light and variable winds ; and during these eight days there was 

 no steady inflow of air from any impure direction, so that the pure air which covered our 

 area in the beginning of the month probably circulated backwards and forwards over our 

 area. Though the lower air would be becoming impure, yet as the winds were light, and 

 being the winter season, the impurity would not ascend to the upper air, so that 

 probably the upper air remained pure. The Meteorological Report shows that during 

 the second week of March, after the winds had fallen, the temperature became exces- 

 sively low, the different stations in Scotland for that week being as much as from 

 8° to 11° below the mean. 



