656 Mil JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



between the number of particles, the humidity and the haze, so that, knowing any two 

 of them, we can calculate the third. 



Dust and Transparency on Ben Nevis. 



We shall now examine the Ben Nevis observations for the periods shown in Dia- 

 grams I., II. and III., and see if we can find in them any information as to the effect 

 of the dust on the transparency at high level. Passing over the 1891 observations, as 

 there are in the report for that year very few observations of transparency, we come to 

 the observations made in 1892, which are more full, and give us some information as to 

 the effect of the dust at that station. It should be kept in mind that we can only get 

 information as to the state of the air at this station on a small proportion of the days. 

 The top of the Ben is so frequently in cloud that for many days in succession 

 no extended view can be obtained. The scale of transparency used at the Ben Nevis 

 Observatory is from to 4 : when in cloud, and 4 when it is at its maximum clearness. 

 All days on which the Observatory was free from cloud will be now referred to. 



On the 4th July the transparency was a maximum, and dust at high and low level 

 was very low, as will be seen from Diagram II. Next clay the clouds settled on the 

 mountain and remained till the 10th ; on that day the transparency was 3 to 4, and the 

 number was low at both levels in the morning. But in the afternoon a bank of haze 

 was seen to the S. and S.E. This bank of haze seems to have arrived at the Ben by 

 7 p.m., as the numbers were then high and continued to rise. On the 12th the air 

 attained a transparency of 3, and the dust was low at both stations. On the following 

 day the air was still as clear, and, as will be seen from the diagram, the number was low 

 at high and low levels, except the afternoon readings at low level. On the 

 16th the maximum transparency was again attained, and an examination of the diagram 

 for this day shows that the dust was very low and continued very low during the whole 

 of the day. At high level the mean number was about 160 per c.c, and at low level 

 about 200 per c.c, so that, taking the day as a whole, it was one of the purest observed. 

 The Ben Nevis report says, "Ireland seen, only a thin haze." As Ireland is 125 miles 

 distant, the air must have been excessively clear to give only a thin haze at that distance. 

 The day following, the 17th, was also very clear, while the diagram at first sight seems 

 to indicate a fair amount of dust. It will, however, be observed that the Ben Nevis high- 

 dust observation is connected with dotted lines showing it to be doubtful, owing, the Ben 

 Nevis report says, to " the presence of tourists on the roof and to a calm," while the high 

 numbers at low level on this clay were due to a purely local S.E. wind which did not 

 blow at high level. The clouds cleared for a short time on the 18th, when the trans- 

 parency was 3, and the amount of dust slight at both levels. On the 20th and 21st the 

 transparency was 3. Only the morning and evening readings at low level can, however, 

 be taken for the 20th, as it was one of the days with abnormal readings. The dust, 

 however, at high level on the 20th and 21st was low all day. All these observations in 



