December 1, 1894.] 



KNOWLEDGE. 



283 



generally that, among anthophilous insects, intelligence 

 keeps pace with the length of the proboscis and other 

 marks of special organization. At this rate we should 

 expect butterflies to excel bees in industry and intelligence. 

 Some of the Lepidoptera are at least as quick at their 

 work as the bee. Mr. Darwin mentions twenty visits per 

 minute for a bee which he timed. One of the quickest 

 workers among the Lepidoptera is MucroqlosKn stelhitiiniin, 

 which was seen by H. Miiller to visit one hundred and 

 ninety-four flowers on difi'erent plants in six and three- 

 quarter minutes — that is, at the rate of 28-7 per minute. 

 Our contention regarding the importance of industry would 

 be borne out, however, even although the Lepidoptera were 

 the less industrious of the two orders. Kelatively to the 

 deeper flowers, which bees rarely visit, they are the more 

 industrious, and this is all we are concerned to maintain 

 in seeking to account for the exclusion of the bee from deep 

 flowers on the score of its inferior industry. 



THE HAZING EFFECTS OF ATMOSPHERIC 

 DUST. 



By Dr. J. G. McPherson, F.E.S.E., Lecturer on 



Meti'orolof/f/ in the Unircrsiti/ of St. Andrews. 



THE results of Mr.. John Aitken's observations 

 during a period of eighteen months, to ascertain 

 the hazing efl'ect of the dust-particles in the 

 air without the aid of the dust-couuter, show 

 that, without counting the number of dust- 

 particles, the transparency of the atmosphere is very 

 much destroyed by the impurities communicated to it 

 while passing over the inhabited areas of the country. 

 He has shown that the thickness of a haze depends on the 

 number of dust-particles present, on the degree of satura- 

 tion of the air, and to some extent also on the vapour- 

 pressure. Supposing we had two samples of air, both at 

 the same temperature, and both having the same depression 

 of the wet-bulb thermometer, if one of these samples be 

 more hazed than the other it will be found to have more 

 dust-particles in it than the other, and to be the thicker 

 the greater the number of particles present. And for a 

 given number of particles, the damper the air the thicker 

 is the haze. These conclusions placed in his hands a means 

 of comparing the amount of dusty impurity in ditt'erenfc 

 masses of air, or of dift'erent airs brought to us by winds 

 from difl'erent directions. • • • 



He took Falkirk for his centre of observations, This 

 town lies a little to the north of a line drawn between 

 Edinburgh and Glasgow, and is nearly midway between 

 them. If we draw a line due west from it, and another 

 due north, we find that in the north-west quadrant so 

 enclosed the population of that part of Scotland is 

 extremely thin, the country over that area being' chiefly 

 mountainous, and there is not a town in it of any size withm 

 70 miles, with the exception of Stirling. In all the other 

 directions the conditions are quite difl'erent. In the north- 

 east quadrant are the fairly weU-populated areas of 

 Aberdeenshire, Forfarshire, and the thickly-populated 

 county of Fife. In the south-east quadrant are situated 

 Edinburgh and the well-populated districts of the south- 

 east of Scotland. And in the south-west quadrant are 

 Glasgow and the large manufacturing towns which surround 

 it. Therefore, Falkirk has round it three thickly-populated 

 areas, while the fourth is very thinly populated. The 

 result is that while the winds from the three districts 

 bring air polluted in its passage over populated areas, the 

 winds from the north-west quadrant come comparatively 



pure. If, now, the air that comes from these several 

 districts be compared, the efl^ects of the products of 

 combustion on the clearness of the atmosphere can be 

 determined. 



Mr. Aitken adopted the plan of estimating the haze by 

 noting the most distant hill that could be seen through the 

 haze. The distance in miles of the farthest away hill 

 visible is then called "the limit of visibility" of the air 

 at the time. But as it is almost never possible to get a 

 suflicient number of hills at difl'erent distances to work in 

 this way, he estimated the amount of haze on some hill 

 at a known distance, and calculated from that estimate 

 the greatest distance at which a hill could be seen under 

 the conditions. For the observations made at Falkirk only 

 three hills are available, one about four miles distant, the 

 Ochils about 15 miles distant, and Ben Ledi about 25 miles 

 distant, all in the north-west quadrant. When the air is 

 thick, only the near hill can be seen, then the Ochils become 

 visible as the air clears, and at last Ben Ledi is seen when 

 the haze becomes still less. After Ben Ledi is visible, it 

 then becomes necessary to estimate the amount of haze on 

 it, in order to get the limit of visibility of the air at the 

 time. Thus, if Ben Ledi be half-hazed, then the limit of 

 visibility will be 50 miles. In this way all the estimates 

 of haze have been reduced to one scale for comparison. 



After going over the hundreds of observations made in 

 his note-book during these eighteen consecutive months, 

 and rejecting all those which were unsatisfactory, from the 

 conditions being uncertain, there remained two hundred 

 observations, which he has classified and arranged for the 

 determination of the hazing efi'ects of atmospheric dust. As 

 the density of the haze could only be compared on those 

 days when the humidity was the same, the first thing was 

 to arrange all the observations in tables according to the 

 wet-bulb depression at the time. Tables were accordingly 

 prepared, in one of which the observations made when the 

 wet-bulb depression was 2° were entered ; and so on for 

 depressions of 3° up to 8^. The difl'erent observations in each 

 table were at the same time entered in such a manner that 

 all those made when the wind was north were pat together, 

 all those when it was north-east next each other, and so on. 



I will now give an abstract of these tables. As the 

 dryness of the air increases, the limit of visibility also 

 increases. When the wet-bulb depression was 2°, the east 

 wind had a limit of 10 miles, and increased to 22 miles 

 when the air was dry enough to give a wet-bulb depression 

 of 8^. The south wind increased from 8 to 32 mUes, the 

 west from 7 to 17 miles, and the north from 50 to 172 

 miles. There is a very great dift'erence in the transparency 

 of the wind from the different directions. In the north- 

 west quadrant the winds made the air very clear, whereas 

 winds from all other directions made the air very much 

 hazed. The winds in the other three areas are nearly ten 

 times more hazed than those from the north-west quadrant. 

 When the wet-bulb depression is 2°, the wind from that 

 quadrant is about 6-2 times clearer than air coming from 

 the best of the other areas ; and when the air is drier than 

 gives more than 2^ depression, the mean of all the 

 observations shows that the air from the north-west 

 quadrant is more than nine times clearer than that from 

 the other directions. That is, the table shows that the air 

 from densely-inhabited districts is so polluted that it is 

 fully nine times more hazed than the air that comes from 

 the thinly-inhabited districts ; in other words, the atmos- 

 phere at Falkirk is about ten times thicker when the wind 

 is east or south than it would be if there were no fires and 

 no inhabitants. 



It may be interesting to show how much the individual 

 observations differed from each other. The limit varies 



