632 MB JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



that the limit of visibility in the morning when looking east was 70 miles, while, looking 

 westwards, it was 250 miles. At this time the number of particles was not great. 

 After mid-day, when the number had become great, the limit of visibility was reduced 

 to about 50 miles ; while in the evening the haze was so thick, looking westwards, that 

 it entirely obscured the lower slopes of Pilatus, and the earth's shadow on the haze was 

 noticed to be particularly well marked at sunset. 



The 14th was a remarkably fine day, — cloudless from sunrise to sunset, and almost 

 too hot in the sunshine for comfort. A southerly wind had sprung up in the night, and 

 continued to blow all day. Under the influence of the southerly wind the purity of the 

 air began to increase, and the number of particles, which was 3300 on the previous 

 evening, fell to 1100 at 7 a.m. The daily maximum at 3 p.m. was 3325, the wind 

 having fallen at that time. It will be noticed that the fall in dust from the high num- 

 bers of the previous day was accompanied by a decrease in the humidity and an increase 

 in the transparency. Further, it will be noticed that there is not the same difference 

 in the transparency when looking east and looking west as was observed on the 

 previous clay. 



The south wind continuing to blow on the morning of the 15th, the sky remained 

 cloudless and the number of particles small, being only 925. Before mid-day the wind had 

 changed to an impure direction, and at 1 p.m. it was north, and the number of particles 

 increased to 4405. The air had also lost its dryness and transparency. During the rest 

 of the day the number of particles was high, the weather changed, clouds formed on the 

 Kulm, and rain fell in the evening. 



The 16th was the last day of this visit. The morning was cloudy, and an occasional 

 cloud passed over the Kulm. The wind was from the impure direction, but it was slight. 

 At 7 a.m. the number of particles was 1225, and it rose to 2650 at 1 p.m. On descend- 

 ing the mountain the air was tested at low level at three different places at some dis- 

 tance from Vitznau. These three tests gave nearly the same result, all of them being a 

 little over 5000 per c.c. 



Sunrise and Sunset Colours on the Rigi. 



In Part I. I have referred to the colours seen on earth and sky, at sunrise and sunset, 

 when viewed from high and low level, and have expressed the opinion that the colours 

 seen at high level have been very much overestimated, and that they are really not so 

 fine as seen at low level, and further reasons have been given why this should be so. I 

 may now say that my experience during these last four visits to the Rigi entirely confirms 

 the opinion formed during the first visit. 



Frequently at high level, though the sun was unclouded, there was very little colour 

 cither on clouds or snow. Sometimes there was some, but it was always of short dura- 

 tion. So soon as the sun was some distance above the horizon all distinct colour 

 disappeared. The greater beauty of the colouring at low level was forcibly impressed on 



