3*2 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



level observations were taken, so that for this interesting period no comparisons can be 

 made. As a rule, the number at the high level was less than at the low one, but there 

 were exceptions to this. For instance, on the morning of the 4th the number was much 

 greater on the Ben than low down. This was owing to the upper station being in a N.E. 

 wind, while the lower one was in a purer N.W. one. Again, on the 24th, under the 

 influence of an easterly wind, the number at the high station was greater than was observed 

 in a N.W. wind at the low station. This tendency of the number to rise at the upper 

 station with easterly winds is also shown in the numbers for other days. For instance, 

 the numbers during the night of the 22nd rose with an E. wind from 225 to 550, and 

 the number which was about 200 on the 25th rose during that night to 1500 by the 

 wind chancrinsj from N. to S.S.E. 



As the observations taken from the 19th to the 25th illustrate the effect of the 

 wind on the distribution of the dust at the two stations, we shall here consider them 

 more in detail. On the 19th, when there were variable light airs, the number of particles 

 was occasionally very high at both upper and lower stations. But a change took place 

 in the wind next day. At 1.30 a.m., when the first observation was made at the high 

 level on the morning of the 20th, a N.N.W. wind had begun to blow, and had swept 

 away the impurities of the previous day, the number having fallen from 2200 to 

 562 per c.c. ; and the number remained about 500 till 6 a.m., when the observations were 

 stopped, and were not resumed till near midnight. When the observations were begun 

 at low level on the morning of the same day, i.e., the 20th, the number was almost 

 exactly the same as it was in early morning at the high level being a very little under 

 500. During the whole of this day the air remained about the same degree of purity at 

 the low level. When work was resumed about midnight at high level the number was 

 very low, as low as 10 per c.c. Early on the following morning, i.e., on the 21st, the 

 number was excessively low at high level, being only 2 per c.c. at 4 a.m., and the number 

 remained very low all day, the day maximum being about 200. At low level the number 

 was also found to have fallen very low. When testing began in the morning the number 

 was occasionally under 50 per c.c, and the maximum during the day only 180. Next 

 day the numbers remained very low at both stations, though not quite so low as on the 

 previous day. On the 23rd matters continued in much the same condition, the wind 

 was still blowing strong from the N.W., and extremely low numbers were observed at 

 both stations. Less than 40 per c.c. was twice observed at the high level, whilst under 

 20 was observed at the low one. The maximum at neither station got much over 250 

 during the whole day. On the 24th the number was under 50 at the high level in early 

 morning, but as the day advanced the number rose to 675, under the influence of the 

 day maximum and change of wind to the E. ; whilst at the low station the numbers 

 never rose over 210, owing to the N.W. wind continuing to blow all day at that level. 

 The numbers which were large during the day at the upper station fell to 200 during 

 the night, under the influence of a northerly wind. At 10 a.m. on the morning of the 

 25th the number of particles at both stations was about 200, and the wind at low level , 



