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



the clouds rose as the clay advanced, a close, drifting rain continued all the afternoon. 

 Under these conditions the number of particles fell to 579 per c.c. 



The morning of the 24th was cloudy, and though the number of particles was low, 

 the air was not clear, owing to it being damp. The number of particles varied 

 greatly from time to time as the day advanced, owing to changing winds, and rose as 

 high as 6400 when the air was coming from the direction of Zug, but fell again in the 

 evening. 



There was a fine opportunity on the morning of the 24th for observing the manner 

 in which the air heated on the mountain slopes rises to the upper regions. While the 

 Rigi was in cloud the sun had been shining brightly on its eastern slopes in the early 

 morning, and the air heated there was rising straight up to the Kulm. This ascending- 

 air clouded as it rose and completely shut out the view from the Kulm in the morning ; 

 but as the day advanced this cloud thinned away, and its gradual formation on the 

 lower slopes and its ascent to the higher levels became visible from the Rigi and formed 

 a most interesting sight. It looked as if the Lake of Zug was a vast caldron from which 

 steam was rising. Owing to the entire absence of wind and only a very slow movement 

 of the upper air from the west, this ascending cloudy air was not interfered with till 

 it rose above the Kulm, when it was caught by the westerly current and carried east- 

 wards. Under most conditions these rising currents on mountain slopes are invisible. 

 In some cases, such as the above, when the condensation begins at a comparatively low 

 level, the upward movement is easily seen, but it has also been frequently observed 

 when the ascending air only began to cloud when it was 500 or 1000 feet above the 

 mountain. 



The sunrise on the 25th was cloudless, the number of particles low, and the air very 

 clear and very dry. As the day advanced the number of particles rose to 1750 at 1 p.m., 

 after which observations were stopped on the Rigi. On descending to Vitznau and 

 testing the air at low level it was found to be not very pure. The number varied at 

 different places, the lowest being 5950 observed on board the steam-boat, and the 

 highest near Vitznau, the air coming off the lake having 7400 particles. The air was, 

 however, clear, partly owing to its being very dry. The air, however, on this occasion 

 looked unduly clear for so much dust, even with the great dryness. It will be noticed 

 from Table II. that the south wind had begun to blow on the Rigi on the 24th and was 

 still blowing at the time these low-level observations were being made on the 25th. 

 Next day, the 26th, the south wind had greatly increased in force and blew strong all 

 day. When the air was tested on the evening of the 26th near Lucerne, it was found to 

 be very pure. It seems, therefore, very probable that the clearness on the 25th, with so 

 high a number of particles at low level, was due to this southerly wind having cleared 

 the upper atmosphere, though at the time the Vitznau observations were taken it had 

 not descended to low level and swept away the impurities near the surface of the earth. 



