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



was mostly southerly, the circulation was a little confused, the winds on the Santis and 

 Rigi being nearly opposed. The upper circulation was evidentty weakening, while at the 

 lower stations north of the Rigi the winds had set in from the N., and at some stations 

 were blowing with considerable force. 



This impure northerly air would seem to have penetrated some distance into the 

 northern valleys of the Alps, and been turned over by the upper southerly currents. 

 Judging from the number of particles, the upper fold of this current would be as high as 

 the top of the Rigi. This folding over of a contrary lower current by an upper one has 

 been frequently observed, both with the dust-counter and by actual observation. On one 

 occasion it was seen very clearly on the Rigi, when the upper wind was from the S. and 

 the lower one from the N. The northerly air low down penetrated some distance into 

 the valleys, where it rose on the mountain slopes and curved upwards. This air being 

 nearly saturated, as it rose on the mountain slopes it condensed much of its moisture, 

 forming a cloud which revealed the directions of its movements. The advanced and upper 

 part of the cloud rose, and was caught by the southerly wind and carried northwards again. 

 If anyone had been testing in that southerly current at the top of the cloud, he would 

 have been testing not southerly but northerly air. A similar condition of matters seems 

 to have prevailed on the 20th. The lower impure northerly wind forced its way into the 

 valleys of the Alps, where it rose on the mountain slopes into the region of the upper 

 current, and was carried by it in the opposite direction. When the air at low level was 

 tested on the afternoon of this day it also was found to be very impure. 



Kingairloch Observations. 



The next observations entered in the table are those taken at Kingairloch about the 

 same time of the year as the observations given in Part I. The first thing that strikes 

 one on looking over the table for 1890 is, that the number of dust particles fell very 

 low on a number of occasions, the lowest being much lower than anything observed in 

 the previous year. Indeed, the lowest numbers are much lower than any given in any 

 previous table, and are the lowest yet observed at any low-level station. Associated with 

 this low number of dust particles was a low temperature, as will be seen by a comparison 

 of the temperatures given in the tables. The weather on the two occasions showed a 

 marked contrast. During the first visit the weather was warm, bright, and sunny ; whilst 

 the July of 1890 will long be remembered as one of the worst experienced for many 

 years, being cold, wet, and windy. 



During the time I was working at the low level, Mr Rankin was taking observations 

 at the Observatory on Ben Nevis as frequently as his many other duties permitted. Ben 

 Nevis is situated in a north-easterly direction from Kingairloch, at a distance of about 28 

 miles. The two stations are not as close as is desirable, but Kingairloch possesses the 

 advantage of being situated in a less locally polluted area than most places nearer the 

 foot of the Ben. 



