482 MR AITKEN ON ATMOSPHERIC DUST. 



gives it a great advantage over most situations nearer the Ben. It was not possible to 

 have the observations taken at the same hours at both stations ; but this was of little 

 importance, as, owing to the distance between the two places, even when the wind was in 

 the line connecting them, the passing air would take a time to travel between them, and 

 simultaneous observations would not therefore have given the state of a vertical column 

 of air at high and low levels. 



The result of these observations taken at high and low levels, details of which are 

 given in the Trans. Roy. Soc. Edin., vol. xxxvii. parts 1 and 2, show that the air at 

 high level has generally fewer dust particles in it than at low level, but not always so ; 

 and further, the observations show that lower readings have been obtained on the Ben 

 than at Kingairloch. On a few occasions no dust has been observed at the high level, 

 whilst the lowest reading obtained at Kingairloch is 16 per c.c. On the other hand, as 

 high readings have been obtained on the Ben as at Kingairloch. 



It may be as well to define what is meant by these very low readings on the Ben 

 when it is stated no dust was observed. It simply means that in the air tested no 

 particles were visible. But as the observations are made by counting the number of 

 drops falling on the millimetre squares of the micrometer, and as only a square of nine 

 square millimetres can be observed, that means that, when no dust was observed in the 

 ten tests usually made, there was not one dust particle in the cubic centimetre of 

 air that came under observation during these ten tests ; but the observations give little 

 support to the idea that there might not have been one particle in a larger quantity 

 of air. 



The observations show that the effect of the direction of the wind is very marked in 

 the amount of dust both at high and low levels. Whenever the wind blew from the 

 N.W. quadrant, the number of particles at both stations was small, and all low 

 numbers were got in air from that quarter. From all other directions the air was less 

 pure, and particularly when it came from the S.E., that is, from the direction in which 

 lie the most populated parts of the country. When the numbers were higher on the 

 Ben than at low level, it was found to be due to the air at the two stations not coming 

 from the same direction. If, for instance, the wind was northerly at low level whilst 

 it was southerly on the Ben, the number of particles was greater at the high than at 

 the low level. 



These results have been confirmed by observations made in Switzerland, on the Rigi 

 Kulm. The dust observations taken by me at that high-level station show that the 

 number of particles was always high when the wind blew from the thickly inhabited 

 districts, and low when the air came from the region of the Alps. 



The effect of sunshine is to give rise to ascending currents of air, and as this 

 ascending air comes from the lower impure stratum of the atmosphere it brings its dust 

 with it. This is shown by the number of particles increasing on the Ben as the day 

 advances, and again falling after the sun's heat has gone. This effect is also shown in 

 the observations on the Rigi Kulm, the numbers beginning to rise early in the day 



