1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
253 
sion of only 2° - 5, and the air was clear. The tests on this day 
also confirm our conclusion regarding the less effect of the humidity 
when the temperature is low, hut the numbers are not extreme 
enough to show it in a marked degree. 
The observations made on the 25th are interesting, as they were 
taken in exceptional conditions, and gave exceptional results. This 
day will long be remembered as one of the stormiest of a very 
stormy January. From the table it will be seen that at no time in 
the day was the number of particles over 1000 per c.c., and it will 
be also noticed that the air was dry, giving a wet-bulb depression 
of fully 5°; and yet, contrary to all previous experience, the air was 
thick, the hills at little over three miles being invisible. As to 
the cause of this thickness with few particles and dry air, it is at 
present difficult to say anything definite. It, however, seems 
possible that the size and the nature of the particles may have had 
something to do with it. The gale would enable the air to hold in 
suspension large particles of dust, and the atmosphere would also 
contain salt spray. It was noticed that the air had a peculiar 
glistening appearance, quite unlike the usual look of thick air. On 
the evening of this day the air became much clearer, due possibly to 
there being fewer salt particles, as the wind had veered, and the air 
had to travel a greater distance over land before reaching the place 
of observation. Part of the greater clearness was doubtless due to 
the increased dryness which took place in the afternoon. 
On the 27th the wind was again high, blowing strong from the 
north-west. The number of particles had fallen to 250 per c.c., and 
the air had become much clearer. 
The 28th is principally remarkable for being the day on which 
was recorded the smallest number of particles yet observed. The 
storms had now passed, and the wind fallen to a gentle air from due 
north. These tests were made on the hill-side to the north of 
Whistlefield. On this occasion there was great difficulty in getting 
clear of artificial pollution, the great purity of the air enabling the 
existence of a house at a distance of half a mile to be easily 
detected. The site selected for the observations had to be changed 
a number of times before one was found free from local impurities, 
from the instrument revealing the existence of dwellings hidden 
away at a distance among the hills. 
