480 



MR AITKEN ON ATMOSPHERIC DUST. 



the small rain-drops that fall on the micrometer in the receiver being counted by means 

 of the lens S. B is the pump for expanding the air in the receiver. C is a filter of cotton- 

 wool for filling the receiver with dustless air, the air being admitted through the stop- 

 cock 0. Water for moistening the air is admitted by the stopcock D from the vessel E 

 by means of the pipe E'. F is a lamp for illuminating the micrometer, the light entering 

 the receiver through the glass cover. G is a water lens for concentrating the light of 

 the lamp on the micrometer. H is a large glass condensing lens which could be used in 

 place of the water lens. The air to be tested was introduced from the outer air by 

 means of the tube 2", which was connected with the instrument by means of the short 

 india-rubber tube K, attached to the tube L on the instrument. The tube I was of 

 such a length that it could be put through openings on any side of the tower, so as always 

 to point in the direction from which the wind at the time was blowing, so as to get air 

 uncontaminated with local pollution. The air to be tested is admitted to the receiver 

 A by the stopcock O, which communicates with the receiver and the small chamber jV 

 by means of a cross passage in the plug of the stopcock. In order that the air in the 

 chamber N should be the same as the outer air, a continuous current was kept up 

 through it by means of the aspirator M, which was connected with N by means of the 

 pipes P P, the stopcock Q being used for allowing the current to flow when a test was 

 to be made. The different proportions of impure to pure air required for making tests 

 were introduced into the receiver A by pumping out, by means of B, certain measured 

 quantities of air, its place being taken by the impure air admitted from the chamber N 

 through the stopcock ; the different proportions of impure to pure air being regulated 

 by means of the bridles R and R', one of which, when raised to a horizontal position, 

 checked the length of the stroke of the pump at the required proportion, enabling 

 exactly the same proportion of air to be measured for each of the ten successive tests 

 made during each observation. 



Dust Particles at Ben Nevis Observatory. 



See " Preliminary Note on the Observations of Dust Particles at Ben Nevis Observatory." By A. Rankin. 

 Journal of Scottish Meteorological Society, vol. ix. p. 125. 



Table I. gives the monthly averages and the highest and lowest observations of dust during the first few 

 months that the dust-counter at Ben Nevis Observatory was in use. 



Table I. — Number of Dust Particles per cubic centimetre. 





Jan. 



Feb. 



Mar. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



1890. 

 Mean, 

 Maximum, 

 Minimum, 







1,518 

 12,862 



175 



3,582 



10,750 



650 



2,526 



4,940 



250 



700 



3,850 



50 



588 

 4,000 



606 



1,286 

 67 







418 



3,150 



12 





1891. 

 Mean, 

 Maximum, 

 Minimum, 





1,515 



6,350 



4 



557 



7,600 







1,018 



14,400 



11 



988 



4,400 



4 

















