486 MR AITKEN ON ATMOSPHERIC DUST. 



seeing how far the pollution due to inhabited districts affected the clearness of the 

 atmosphere. When the haze observations were made, a note was also at the same time 

 taken of the direction of the wind, and of the wet bulb depression. About two hundred 

 observations were used for working out the results. These observations were first arranged 

 in tables for each direction of the wind, and, for the reasons given, these were again 

 separated into other tables at the different wet bulb depressions. The result of this 

 investigation showed that whenever the air came from anywhere from north of west to 

 due north the air was always very clear, and showed the maximum limit of visibility. 

 When the air came from the N.E. quadrant it was much more hazed, and when from all 

 directions with any south in it the air was densely hazed. With the wind in the N.W. 

 quadrant and a wet bulb depression of from 3° to 4° the limit of visibility was about 

 one hundred miles, and with a wet bulb depression of 8° the limit was 200 miles or 

 more, whereas when the wind was southerly the limit of visibility was, with a wet bulb 

 depression of 3° or 4°, only from eleven to fifteen miles, and with a wet bulb 

 depression of 7° or 8°, it was only from fifteen to forty-five miles. 



A map of Scotland was also prepared, and on it the different counties were shaded 

 according to the density of the population inhabiting them. Round Falkirk were drawn 

 curves, or isatmid lines, that is, lines of equal clearness, showing the limits of visibility 

 in the different directions when the wet bulb depression was 2°, 3° and 4°, 5° and 6°, and 

 7° and 8°. An examination of this map shows that there is a close relation between the 

 transparency of the air and the density of the population, due no doubt to the fires, 

 furnaces, etc., which throw their impurities into the atmosphere. From Falkirk as a 

 centre, in all directions between due west and due north, the whole quadrant is but 

 thinly populated, and the isatmid lines over this area open out to a very wide extent, 

 and extend beyond the limits of the Western Isles when the air is dry. In the N.E. 

 quadrant all the lines come much closer to the place of observation, while in all directions 

 to the south the lines approach very close to Falkirk, and the lines approach the closer 

 the deeper the shading on the map in the direction. That is, the denser the population, 

 the more the air is hazed. 



Summing up the general results, it is found, first, that with W.N.W. and N. winds 

 the air is six times clearer than with southerly winds when the air is damp, and nine 

 times clearer when it is dry. The air at Falkirk is ten times more hazed when the 

 wind is in the E.S.E., S., and S.W., than it would be if Scotland were a desolate island 

 in mid-ocean. Third, the transparency of the atmosphere increases with the dryness, 

 being 37 times clearer when the wet bulb depression is 8° than when it is 2°, a result 

 similar to that obtained from the observations made with the dust-counter. 



Passing now to the relative amount of dust in the atmosphere of different parts of 

 the world. As yet but few observations have been made out of Europe, but so far as 

 they go they show that the air of the Western Highlands of Scotland is by far the least 

 dusty of any in the world. Dr Fredlander in 1894-95, while on a tour round the world, 

 made a number of dust observations, but at no part of his journey did he find any air 



