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



among the hills, with high mountains to the N., the direction was often quite local. 

 From an examination of my weather tubes taken at Mentone at the time, I find that 

 the true wind on the 14th and 16th was northerly, and not S.E. or E. as observed 

 among the hills. 



During the time of the previous observations at Bellagio the weather was always dull 

 and the amount of dust great, but on this occasion there were opportunities of testing clear 

 air. It will be observed from the table that whenever the wind fell or went southward 

 at this station that the number of particles was great ; and, on the other hand, when it 

 went northerly, or became strong, the number of particles fell. 



In the later observations I have tried to introduce a more definite measure of the 

 clearness of the air than that given in the table in Part I. For this purpose the limit of 

 visibility, or the extreme distance at which a mountain could be seen, has been used. If 

 there were any mountain sufficiently far away to be just visible, then the distance of that 

 mountain was the limit of visibility for the air at the time. At most stations, however, 

 mountains are not available at sufficient distance, and the hazing effect on near ones has to 

 be estimated, and from this estimate the extreme limit of visibility is calculated. This 

 plan, however, is not very satisfactory, as the air is not equally clear in all 

 directions, being greatly influenced by the position of the sun. At mid-day, two hills 

 equally distant, but one N. and the other S. of the observer, will not look equally 

 hazed; the one to the N. always looks much clearer, or, stated generally, there is 

 always more haze when looking in the direction of the sun than when looking away from 

 it. It is for this reason that the direction of visibility requires to be known as well as 

 the distance. 



It will be seen from the Bellagio observations given in the table that with over 6000 

 particles, and a depression of the wet bulb of from 5° to 8°, nothing could be seen beyond 

 a limit of 15 miles looking in the direction of the sun, though in the opposite direction 

 hills could be seen to a much greater distance. On the other hand, when the number 

 of particles fell to about 1000, while the humidity remained the same, the air was 

 clear, and all hills within range could be seen, and only some haze between the observer 

 and hills 1 5 miles distant. The relation between the transparency and the humidity 

 of the air, which has been pointed out in Part I., is also clearly seen in these Bellagio 

 observations. Increase in the number of particles, if accompanied by constant humidity, is 

 in a general way accompanied by a decrease in transparency ; and increase in the humidity 

 is also accompanied by a decrease in the transparency if the number of particles remains 

 constant — i.e., both dust and humidity tend to decrease transparency. These con- 

 clusions can, however, only be looked for in a general way from observations taken at a 

 place of this kind, where it is difficult to get air for testing which is free from local 

 pollution. 



There are no points of special interest in the Baveno observations, nor in those taken 

 at the entrance to the Simplon Pass. There was no bright, clear air while the observations 

 were being made ; the quantity of dust was always large and the air thick. The wind 



