lOO 



NATURE 



[January 28, 1892 



pointed out that the violent down-rush of air produced by 

 a heavy hail-shower at the time would bring down the 

 purer upper air to the place of observation, so that the 

 air tested at 7.10 p.m. was not the same as that tested 

 previously, but was air from a higher and purer stratum. 

 The purifying influence of the down-rush of air in this 

 case was not nearly so great as was observed in the 

 heavy rainfall on the Eiffel Tower recorded in Part I. 



By midday of the last day of the visit to the Rigi Kulm, 

 the air again became very much hazed, and the number 

 of particles rose to about 10,000 per c c. On descend- 

 ing the mountain, the air was again tested on this day, at 

 the level of the lake, at 3 p.m. Here the number was 

 a little over 10,000, or very much the same as it was when 

 tested on the way up. Its humidity was also the same, 

 and it had the same thick appearance. 



Just when on the point of finishing the tests at the 

 level of the lake, it was observed that the numbers were 

 becoming unsteady and were falling. The tests were 

 therefore continued for a considerable time longer, when 

 it was found that the dust particles, which at first were 

 10,250 per c.c, gradually decreased to 1700 per c.c. ; a 

 most unusual experience, and one which might have 

 shaken our confidence in the dust-counter, had it not 

 been noticed that this decrease in the dust was at the 

 same time accompanied by a rise in the temperature of the 

 air, and by a decrease in its humidity. When the tests 

 were begun, and the number of particles was great, the 

 temperature was 71°, and the wet bulb was depressed 11 

 degrees ; but when the amount of dust was small, the tem- 

 perature had risen to 74''"5, and the wet bulb depression was 

 as much as 18-5 degrees. These observations all show that 

 in a very short time there had been an entire change of the 

 air at the place of observation. This result is shown to 

 have been produced by the local wind, which at first blew 

 in from the lake, changing by the upper part of the 

 current striking the nearly perpendicular face of the 

 mountain, curving downwards, and then blowing out 

 towards the lake, thus bringing a purer upper air to the 

 place of observation. After a time the down-blow 

 ceased, and the wind at the level of the lake returned to 

 its original direction. After the wind had blown a short 

 time off the lake, the number of particles rapidly in- 

 creased, and became slightly higher than it was at first. 

 The temperature and humidity also returned to near their 

 original readings. 



The Rigi Kulm observations show the daily maximum 

 of dust very clearly. On all the days, except one, the num- 

 ber of particles was least in the morning, and increased 

 greatly as the day advanced, owing to the ascent of valley 

 air on the sun-heated slopes of the mountain. The im- 

 pure air had generally arrived at the mountain top before 

 midday, and by midday the number was generally three 

 times greater than it vvas in the morning. 



Observations were also made on one day on Pilatus 

 Kulm. During the whole day the mountain was covered 

 with cloud. On this occasion the numbers were found to 

 be very unsteady, varying greatly in short intervals. 

 The highest reading was 1275 per c.c, and lowest 625 

 per c.c. 



The result of an investigation into the cause of the 

 difference observed in the air of Switzerland on the two 

 occasions is then given. It is shown that during the 

 days of the first visit the upper air circulation was 

 generally from the south, and was pure, as it came from 

 the uninhabited area of the Alps ; while during the first 

 days of the second visit the general air circulation was 

 northerly — that is, from densely inhabited and polluted 

 areas. While this northerly circulation continued, the air 

 was thickly hazed, and the number of particles was great. 

 On the morning of the day of the thunderstorm already 

 referred to, the wind had changed and begun to blow 

 freshly from the south, at the St. Gothard. It had also 

 changed to south on the Siintis and Rigi. But though 



NO. I 16 I, VOL. 45] 



the wind had begun to blow from a piire direction in the 

 morning, it was evening before the pure air arrived at the 

 Rigi. It was at the meeting area of the lower impure 

 northerly air with the upper pure southerly that the 

 thunderstorm took place. The storm began to the south, 

 where the currents first met, and travelled northwards 

 as the pure south wind drove the northerly air before it. 

 The sudden drop in the number of dust particles in the 

 storm may therefore possibly have been due to the arrival 

 of the pure southerly air. 



The day after the storm the southerly wind continued 

 to blow, the air became very clear, and the number 

 of particles fell practically as low as was observed on the 

 previous visit, while the air had much the same clear ap- 

 pearance it had on that occasion. It would thus seem 

 that the clearing of the atmosphere on this occasion, 

 though at first sight it may have appeared to have been 

 caused by the thunderstorm, was in reality caused by a 

 change in the circulation of the air. The clearing would 

 thus appear to have been due, not to a clearing of the 

 dust out of the air, but to a change of the air itself. 



The observations made on Ben Nevis and at Kingair- 

 loch, for July 1890, are then discussed, and a comparison 

 made of the variations in the amount of dust at high and 

 low levels. The observations made at Kingairloch in 

 1890 confirm the conclusions arrived at in Part I., that 

 the air at this station has most dust in it when the wind 

 blows from the east, south-east, and south— that is, from 

 inhabited areas — and less when the wind blows northerly 

 — that is, from uninhabited areas. Some very remark- 

 able exceptions to the latter conclusion were, however, 

 observed. On a number of days when the wind was 

 northerly the number of particles rose high at some hour 

 of the day. On examining into the cause of the excep- 

 tional readings with northerly winds, it was found that 

 they almost always occurred when the isobars over our 

 area were irregular and the general circulation in a 

 confused condition, and blowing in different directions at 

 different stations at no great distances from each other. 

 It is suggested that under these conditions uniformity in 

 the air cannot be expected ; that, while testing in a northerly 

 wind, it may be southerly air that is being examined. In 

 confirmation of this it is shown that whenever the general 

 air circulation over our area was mixed and irregular, high 

 numbers were also observed at some time of the day on 

 Ben Nevis. 



An examination of the numbers of dust particles shows 

 that there was much less dust in 1890 than in 1889. The 

 numbers in 1 890 fell very low on many days, and extremely 

 low on a few days. The lowest number observed was 

 i6"5 per c.c. This number is much lower than any 

 previously observed at any low-level station. Associated 

 with the small amount of dust was an exceptionally low 

 temperature. July of 1890 will long be remembered as 

 one of the most inclement experienced for many years, 

 being cold, wet, and windy. 



A comparison is made of the amount of dust at Kin- 

 gairloch and Ben Nevis. Although there is a considerable 

 resemblance between the figures at the two stations, yet 

 the likeness is not very close. This is owing to the daily 

 maximum of dust which takes place at high levels on most 

 days ; also to the effect of the wind on the amount of dust 

 not being the same at high and low levels ; and further, 

 the directions of the winds are not always the same at both 

 stations. As a rule, there was much less dust at the high 

 station than at the low one, and when the dust increased 

 at the low station it also generally increased at the high 

 one, and vice versa. These rules, however, are not without 

 exceptions. Sometimes the lower air was purer than the 

 upper ; this happened when the wind blew from a pure 

 direction low down, while it was from the east or south 

 on the Ben. Mr. Rankin has shown, from an extensive 

 series of observations at the Observatory, that the south, 

 south-east, and east winds bring the most impure air to 



