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



tion. On looking downwards to the valleys and lakes, the air was thick and black 

 looking. Sometime before sunset the air was so thick to the westwards that the lower 

 slopes of Pilatus were scarcely visible. The lake to the south was visible through a thick 

 haze ; and the haze was also distinctly seen when the eye w T as turned in a direction at a 

 level with the top of the mountain. Between the observer and the mountains it appeared 

 as if a veil were hung between him and the distant scenery. 



Sometime before sunset this hazy veil became coloured by the rays, of the setting 

 sun, and its upper limit was well defined in the eastern sky at an elevation considerably 

 above the highest Alp. At sunset the dusty impurity became still more apparent as the 

 earth's shadow crept up its lower edge. The shadowed part looked bluer by contrast 

 with the red haze above, and where the upper edge of the veil mingled with the blue of 

 the sky, it passed by imperceptible degrees through white, till it was lost in the blue of 

 the heavens. 



Although the sun set on this evening in a cloudless sky, it looked more like a 

 harvest moon than the orb of day, and was so dull that it could be gazed at without the 

 slightest discomfort. Its rays after penetrating the thick air were so feeble that no colour 

 effects were seen on the mountains ; as they were not strong enough to give any percep- 

 tible direct illumination, while there was much diffused light reflected by the dust-laden 

 air. 



The air on the 16th continued much the same as it was on the previous day, only if 

 anything thicker in the afternoon. As the wind rose a little on the afternoon of this day, 

 its increase may have been one of the reasons for the increase in the number of particles 

 observed as the da} 7 advanced. At low levels increase of wind is accompanied by a decrease 

 in dust. As a natural sequence, an increase in wind gives rise to an increase in dust 

 at high levels. This will be the case at least when the wind begins to blow and mixes 

 the lower impure air with the purer air above ; but after the winds have blown for some- 

 time, and cleared the lower impurity away, the amount of dust at high levels will fall again. 



During the 17th the wind remained much the same as on the previous two 

 days; the same thick pall hung over the lower landscape and veiled the hills in the 

 distance, the number of particles remained high, and the air fairly dry. During 

 the afternoon the clouds on the distant mountains began to clear away, but 

 about 5.30 p.m. a large mass of clouds formed in the S.W. This bank of clouds 

 gave rise to a fine thunderstorm, with a good deal of lightning and rain, which passed to 

 the W., but did not come near the Rigi. About 6.30 P.M. another fine mass of clouds 

 streamed in from the S., filling the sky to the S.E. On the N. also there lay large 

 masses of thunder-clouds. The storm-clouds to the E. and W. of the Eigi passed 

 northwards, while the lower wind was directly opposite. The sun set on this evening 

 amidst grand towering masses of thunder-clouds, which filled the sky in all directions, 

 save over the place of observation. By 9 p.m. these clouds had all passed away to the 

 N., the wind had fallen, and the stars shone in a clear and tranquil sky. 



The 18th was a day of special interest. The morning opened much the same as the 



