(324 MB JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



hour later it had fallen to 3900. On return to low level it was found to be still lower, 

 being 2250. This gradual clearing at high and low level was due to the wind rising and 

 beginning to blow from a purer direction. In these cases the wind was blowing in such 

 a direction that the valley air was forced up the mountain slopes and was therefore but 

 little purified, being only slightly mixed with the higher and purer air. On the 11th the 

 conditions were different. At low level the number was 3550, and it remained at that 

 figure up to an elevation of 1200 feet. At this situation the air was coming directly up 

 from the valley. On ascending the mountain 200 feet higher a current blowing from the 

 mountains was met. In this mountain air the number suddenly fell from 3500 at 1200 

 feet to 875 at 1400 feet, and to 840 at 1800 feet. The conditions on the 12th, 13th and 

 14th were similar to those of the 9th and 10th. Though observations were made up to 

 an elevation of 1800 feet, no great improvement was observed in the purity of the air, 

 the numbers being much the same as observed at low level on return. As in the previous 

 cases, this was due to the direction of the wind being such as to drive the valley air up the 

 mountain slopes. On the 14th the number at low level was 4700, and fell only to 3750 

 at 1200 feet, to 3400 at 1500 feet, and to 3150 at 2000 feet, the air at all the different 

 levels on this day also coming directly up from the valley. 



On going up the lake in the steam-boat on my way northwards on the 16th, as on 

 the previous year a thunderstorm was in progress in the distance and some rain fell. 

 The numbers were 3400 before and 1950 after the storm. 



Coming to the observations at different levels made in 1893, it will be seen that on 

 the 29th April the number at low level was 3500, and fell to 1850 at 1500 feet. On the 

 1st May, owing to the wind bringing the valley air up the mountain, the number fell 

 very little, only from 2800 at the lake side to 2300 at 1500 feet. On the 2nd the air 

 was still coming up off the lake side, and the numbers fell from 6400 to 4500 at 1500 

 feet. On the 4th the number was 5100 at low level on starting, and it fell to 1900 at 

 1500 feet and to 1525 at 2000 feet; and on return to the lake side it was 3350, which 

 was lower than when tested before starting, due to a rise in the wind. On this occasion 

 the fall in numbers was very well marked. On the 5th there was no great difference in 

 the numbers at high and low level ; but on this occasion, when the air was tested on 

 return to low level, it had become greatly purified by a strong wind which had suddenly 

 sprung up. On the 6th at low level the number was 4800, and fell to 1650 at 2000, but 

 the number was variable, owing to wind frequently changing its direction. On returning 

 to low level on this occasion the number was lower than at 2000 feet, owing to a strong 

 wind having sprung up, which reduced the number to one-fourth of what it was when 

 tested before going up. 



If we put the observations at different levels into tabular form, we shall see more 

 clearly the purer condition of the atmosphere at the higher levels. For reasons easily 

 understood the observations have been arranged in two sets (see Tables IV. and V.). In 

 Table IV. are entered the observations taken when the wind was blowing the lower impure 

 air up the mountain slopes, and in Table V. those taken when the wind was along the 



