660 



MB JOHN A1TKEN ON THE NUMBER OF DUST PARTICLES TN THE 



as the extreme limit, all very pure air lias been put at that figure. These tables, 

 however, suggest that this limit is too low by about one-half for exceptionally pure air. 



Coming now to the observations made when the wet-bulb depression was 7° and 

 over, the values of C in this case are far from being alike. The Kingairloch figure is 

 140,028, while the Alford one is only 124,921, or 11 per cent. less. In this case the 

 greater purity of the air is not responsible for the difference. The probable cause is the 

 greater humidity of the Alford observations. In all the Alford observations, with the 

 exception of those made on the 29th and 19th September 1891, the wet-bulb depression 

 was scarcely over 7°, and on these days the value of C was high, while in the Kingairloch 

 table the mean wet-bulb depression of all the observations was greater than in the 

 Alford table. 



Callievar. 



"When at Alford four ascents were made of Callievar, once in 1889, once in 1890, and 

 twice in 1892. Callievar is situated at a distance of 5 or 6 miles from Alford, and is 

 1747 feet high. The state of the air on the occasions of these four visits was very 

 different. It will be seen from Parts I. and II. on this subject that on the occasion of 

 the first visit the number of particles was small and the air clear, all the distant 

 mountains being visible ; and on the occasion of the second visit none of the distant 

 mountains were visible and there was much dust in the air. As will be seen from Table 

 II., on the day the first visit was made in 1892 the air was very thick and the 

 number of particles very great ; but when the second visit was made the following clay 

 the air was remarkably clear, all the distant mountains very distinct, and the number of 

 particles was small. Unfortunately, on the first visit no observations were made of the 

 temperature or the humidity, so that the dust observations on that day are of little value. 



Let us now compare these observations on the hazing effect of the dust made on 

 Callievar with those made at low level, and see how far they agree. In Table XVIII. are 

 arranged the Callievar observations in the same manner as has been done for the 

 Kingairloch and Alford ones, only in this case, as there are so few observations, they have 

 been arranged in one table, and the wet-bulb depression is given in a separate column. 



TABLE XVIII. — Shoiving the Relation between the Numler of Dust Particles and the Transparency 

 of the Atmosphere on Callievar at different Wet- Bulb Depressions. 



Date. 



Wet-Bulb 

 Depression. 



Mean 

 Number. 



Limit of 



Visibility in 



Miles. 



C. 



24th September 1892 .... 



9th „ 1889 .... 



22nd „ 1890 .... 



23rd „ 1892 .... 



6-5 

 1 



4-2 



4-2 



196 



387 

 1184 

 1283 



250 



100 



30 



20 



49,000 

 38,700 

 35,520 

 25,660 



