036 



MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



Dumber of particles great, the air was only once clear, and on that occasion the number 

 of particles was small. 



We can, however, show the effect of dust on the transparency in another way, by 

 selecting the days on which the air was exceptionally clear, and noting the number of 

 particles at the time. For this purpose we shall take the conditions of the atmosphere 

 when Hochgerrach was visible. See Tables I., II. and III. ; also Tables in Parts I. and II. 

 Hochgerrach is about 70 miles from the Eigi in an easterly direction. It is the most 

 distant mountain visible from that situation. Its visibility, therefore, may be taken as a 

 good indication of great clearness of the atmosphere at the time. An examination of the 

 observations for the five visits shows that this mountain was only visible when the number 

 of particles was small. From my notes I find it was visible on thirteen occasions : some- 

 times it was visible for a whole day at a time, but often it was only seen in the morning 

 or evening. Table VIII. shows in the first column the number of times it was seen, in 

 the second the amount of haze between the observer and the mountain on these occa- 

 sions, in the third column the number of particles in the air at the time, and in the fourth 

 the wet-bulb depression. 



Table VIII. 



Number of 

 times Hoch- 

 gerrach was 

 seen. 



Amount of Haze on 

 Hochgerrach. 



Number of Particles. 



Humidity. 



8 

 2 

 3 



I 

 Just visible. 



326 to 850 

 1375 to 1375 

 1825 to 2050 



3° to 10° 

 6°-5 to 8° 

 4° to 6° -5 



It will be seen from this table that on eight of the occasions when Hochgerrach was 

 visible the haze was only ^ to \, and the number of particles was a minimum for this 

 station, and that as the haze increased the number of particles also increased, till at last, 

 with a little over 2000 particles, the mountain became invisible. The humidity of the 

 air will of course have a considerable effect ; but it will be seen from the table that Hoch- 

 gerrach was seen when the wet-bulb depression was small, when the number of particles 

 was also small, and that the haze increased with the amount of dust though the wet-bulb 

 depression showed the air to be dry. 



Of the observations made in these five years, the days in 1889 were by far the clearest 

 and purest, the wind being steadily from the south during the time. The days in 1891 

 were also pure, but those in the three other years were all rather impure during most 

 of the time, owing to the wind being from an impure direction or unsteady. On the 

 days on which the number of particles was small, the dust-veil was thin and its upper 

 limit ill-defined, while on the days the number was great, the dusty impurity was easily 

 seen thickening the air, and colouring the atmosphere at sunset to a height high above 

 Hi'; highest Alp. 



