22 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



there had been but few storms during the period of these observations. I therefore 

 made the most of the opportunity which presented itself while the violent storm of the 

 18th was going on. The occasion was a highly favourable one for getting information 

 on the effect of the electric discharge on dust, as the observations would be made 

 in the very centre of the storm, and in the air in which the lightning discharges were 

 taking place ; and I therefore resolved to take the observations that evening, and with as 

 much accuracy as the difficult and trying conditions would admit of. 



The observations made on the 17th as well as those of the 18th bear directly on this 

 important point, as on both days there was a good deal of thunder and lightning. Now, 

 though there was a good deal on the 17th, yet there was no indication of any reduction 

 in the amount of dust. It is true that in this case the thunder was somewhat distant, and 

 took place in the evening, while the air was not tested till next morning. 



The storm of the 18th, however, is open to none of these objections. From an 

 examination of the numbers in the table, one might at first sight be inclined to conclude 

 that in this case there was evident proof of the reduction of the dust by the storm. The 

 number of particles at mid-day was nearly 4000 per c.c, at 6 p.m. it was still as high as 

 3000, but just in the middle of the storm the number suddenly fell to about 800. In my 

 notes I find a remark to the effect that the observations made during the storm may not 

 be very accurate, owing to the conditions under which they were made. For instance, 

 the 6.20 p.m. observations were taken when the storm was near its worst, and had to be 

 made in the open, as it was too dark to work under shelter ; and at this time the 

 lightning and thunder were excessively near and violent, and the hail came down in 

 heavy showers which obscured the lens of the dust-counter, and made accurate counting 

 almost impossible. Under these conditions only five tests could be made, of which the 

 one given is the average, when a rush had to be made for shelter. The 7 p.m. observa- 

 tions may not be correct from the small amount of light at the time. Although I have 

 thrown some doubt on these observations from the conditions under which they were 

 made, yet, as the number was much the same next morning, there does not seem to be 

 any reasonable cause for supposing they are not fairly correct. If there be any error, the 

 number may probably be too low owing to some of the drops escaping detection in the 

 feeble light at the time. 



But supposing that these observations are correct, and that as low a number as 800 

 was observed after the storm, do they prove that the storm, as a thunderstorm, had any 

 effect on the amount of dust in the atmosphere ? I think it must be admitted they do 

 not. The violent hail-shower falling at the time of the observations would produce a 

 downrush of upper air, and displace the impure air on the mountain by a purer air from 

 above. The purifying influence of the downrush of air produced by the hail in this case 

 was not nearly so great as that observed in a heavy shower of rain on the Eiffel Tower 

 recorded in Part I., when the numbers fell from a very high figure to 226 per c.c. The 

 purifying influence of such downrushes depends chiefly on the purity of the air in the 

 upper region from which the air is carried by the shower. 



