MR AITKEN ON ATMOSPHERIC DUST. 



481 



The means for March, April, and May of 1890 are not comparable with the rest, as the observations in 

 these months were all taken during the daytime and in fine weather. In February 1891 readings were 

 only taken during the last eight days of the month. Taking the average of the remaining months, the 

 mean number of dust particles is 696 per cubic centimetre. The largest number observed was 14,400 per 

 c.c. at 13 hour on 11th April 1891. Hourly readings were taken on the 11th and 12th April 1891, on 

 both of which daj's the number of particles was great : the values for each hour are given in Table II. 

 This table shows how rapidly the number of dust particles fluctuates. 









Table 



II. — Dust Particles per c.c. 













Hour. 



1 



2 



3 



4 



5 



6 



7 



8 



9 



10 



11 



12 



April 11, . 

 „ 12, . . . 



1,600 

 400 



162 



1,400 

 300 



850 

 350 



1,600 

 300 



800 

 275 



450 

 288 



350 



625 



288 



712 

 350 



838 

 225 



2,250 

 325 



















Hour. 



13 



14 



15 



16 



17 



18 



19 



20 



21 



22 



23 



Mid. 



April 11, , 

 „ 12, . 



14,400 

 338 



11,200 

 1,100 



13,500 

 1,200 



9,500 

 1,800 



12,000 

 2,450 



6,500 

 4,400 



4,200 

 2,400 



1,750 

 1,850 



700 



850 

 1,550 



950 

 1,850 



600 

 2,750 



Regular observations were made eight times a day for the three months March, April, and May of 1891, 

 the hours of observation being, 1, 4, 7, 10, 13, 16, 19, and 22. A few breaks occurred in the record, but 

 not sufficient to prevent a fairly correct average for each hour being obtained. These averages are given in 

 Table III., with the mean for each month, the mean of the three months at each hour, and the differences 

 of these hourly means from the general mean of the whole period. 



Table III. — Dust Particles per c.c. 



Hour. 



1 



4 



7 



10 



13 



16 



19 



22 



Mean. 



1891 





















March, .... 



421 



310 



301 



656 



762 



833 



701 



470 



557 



April 



809 



547 



743 



505 



1218 



1912 



1190 



1221 



1018 



May, .... 



978 



720 



666 



491 



869 



1568 



1213 



1397 



988 



Mean, .... 



736 



526 



570 



551 



950 



1438 



1035 



1029 



854 



Differences, 



-118 



-328 



-284 



-303 



+ 96 



+ 584 



+ 181 



+ 175 





This table indicates a well-marked diurnal range in the amount of dust, there being a maximum about 

 the warmest part of the day and a minimum in the early morning. This is mainly due, no doubt, to 

 ascending currents of heated air bringing up dust from the lower atmosphere during the middle of the day. 



The discussion of the observations taken in later years is not yet completed. 



During about a month, generally in July, of the years 1890-91-92-93, simultane- 

 ous tests of dust were made at the Ben Nevis Observatory, generally by Mr Rankin, 

 and by myself at Kingairloch, in order to find the comparative purity of the air at high 

 and low levels. Kingairloch is situated at sea level, at a distance of about 28 miles in a 

 south- westerly direction from Ben Nevis. A situation nearer the Ben would have been 

 preferable, but the great isolation of Kingairloch, and its freedom from local pollution, 



EOY. S0C. TRANS. EDIN. — VOL. XLII. 3 Q 



