F^b. 27, 1890J 



NATURE 



395 



made on the top of La Croix des Gardes. The number 

 here varied from 1 550 per cubic centimetre, when the wind 

 was from the mountainous districts, to 150,000 when it 

 came from the town. 



At Mentone the number varied from 1200 per cubic 

 centimetre in air from the hills to 7200 in the air coming 

 from the direction of the town. 



Tests were made of the air coming towards the shore 

 from the Mediterranean at three different places — at La 

 Plage, Cannes, and Mentone. In no case was the amount 

 of dust small. The lowest was 1800 per cubic centimetre, 

 and the highest 10,000 per cubic centimetre. 



Observations were also made at Bellagio and Baveno, 

 on the Italian lakes. At both stations the number was 

 always great — generally from 3000 to 10,000 per cubic centi- 

 metre. This high number was owing to the wind, during 

 the time of the observations, being light and southerly — 

 that is, from the populous parts of the country. Smaller 

 numbers were observed at the entrance to the Simplon 

 Pass and at Locarno, at both of which places the wind 

 blew from the mountains when the tests were being made. 



A visit of some days was made to the Rigi Kulm. On 

 the first day, which was May 21, the top of the mountain 

 was in cloud, and the number of particles was as low as 

 210 per cubic centimetre. Next day the number gradually 

 increased to a little over 2000 per cubic centimetre, after 

 which the number gradually decreased till on the 25th 

 the number was a little over 500 per cubic centimetre at 

 10 a.m. On descending the mountain to Vitznau the same 

 day, the number was found to be about 600 per cubic 

 centimetre at midday, and in the afternoon at a position 

 about a mile up the lake from Lucerne the number was 650 

 per cubic centimetre. 



Most of the observations taken of Swiss air show it to 

 be comparatively free from dust. This is probably owing 

 to the vast mountainous districts extending in many 

 directions. It is thought that much of the clearness and 

 brilliancy of the Swiss air is due to the small amount of 

 dust in it. 



Owing to the kindness of M. Eiffel an investigation of 

 the air over Paris was made on the Tower on May 29. 

 The day was cloudy and stormy, with southerly wind. 

 Most of the observations were taken at the top of the 

 Tower, above the upper platform, and just under the 

 lantern for the electric light. The number of particles 

 was found to vary very rapidly at this elevation, showing 

 that the impure city air was very unequally diffused into 

 the upper air, and that it rose in great masses into the 

 purer air above. Between the hours of 10 a.m. and i 

 p.m. the extreme numbers observed were 104,000 per 

 cubic centimetre and 226 per cubic centimetre. This 

 latter number was obtained while a rain-cloud was over 

 the Tower, and, as the shower was local, the descending 

 rain seems to have beaten down the city air. The low 

 number continued some time, and was fairly constant 

 during the time required for taking the ten tests of which 

 the above low number is the average. 



The air of Paris was tested at the level of the ground 

 on the same day, the observations being made through 

 the kindness of M. Mascart in the garden of the 

 Meteorological Office in the Rue de I'Universitd. The 

 number on this day varied from 210,000 to 160,000 per 

 cubic centimetre. 



Very few tests have been made of the air of London. 

 The air coming from Battersea Park, when a fresh wind 

 was blowing from the south-west, on June i, was found 

 to vary from 1 16,000 to 48,000 per cubic centimetre;. The 

 numbers observed in cities are of no great value, as so 

 much depends on the immediate surroundings of the 

 position where the tests are made ; so that, while no 

 ow number can be observed, a very high one can always 

 be obtained. Those recorded were taken where it was 

 thought the air was purest. 



Observations have been made in Scotland for periods 



of two or three weeks at three stations — namely, at Kin- 

 gairloch, which is situated on the shore of Loch Linnhe, 

 and about fourteen miles to the north of Oban, at Alford 

 in Aberdeenshire, the observations being made at a 

 distance of two miles to the west of that village, and at a 

 situation six miles north-west of Dumfries. 



At Kingairloch the number varied from 205 per cubic 

 centimetre to 4000 per cubic centimetre. At Alford from 

 530 to 5700 per cubic centimetre, and at Dumfries from 

 235 to 11,500 per cubic centimetre. These three stations 

 were in fairly pure country air — that is, pure as regards 

 pollution from the immediate surroundings. 



Tests were also made of the air on the top of Ben 

 Nevis on August i, when the number was found to be 

 335 per cubic centimetre at i p.m., and 473 two hours 

 later. On the top of Callievar, in Aberdeenshire, on 

 September 9, the number was at first 262, and rose in two 

 hours to 475 per cubic centimetre. 



The pollution of the earth's atmosphere by human 

 agencies is then considered, and it is pointed out that, 

 while on the top of the Rigi and in the wilds of Argyll- 

 shire air was tested which had only a little over two 

 hundred particles per cubic centimetre, nqar villages the 

 number goes up to thousands, and in cities to hundreds 

 of thousands. The increase, though great, is shown not 

 to be in proportion to the sources of pollution, and it is 

 pointed out that part of this is owing to the impure 

 stream of air being deepened as well as made more 

 impure. 



About 200 particles per cubic centimetre is the lowest 

 number yet observed, but we have no means of knowing 

 whether this is the lowest possible, or of knowing how 

 much of this is terrestrial and how much cosmic, formed 

 by the millions of meteors which daily fall into our atmo- 

 sphere. Even in the upper strata there seems to be dust, 

 as clouds form at great elevations. 



The effect of dust on the transparency of the atmo- 

 sphere is then discussed with the aid of the figure in the 

 table. It is shown that the transparency of the atmo- 

 sphere depends on the amount of dust in it, and that the 

 effect of the dust is modified by the humidity of the air. 

 With much dust there is generally little transparency, but 

 it is pointed out that air with even 5000 particles per c.c. 

 may be clear, if it is so dry as to depress the wet-bulb 

 thermometer 10° or more. By comparing days on which 

 there was the same amount of dust, it is seen that the 

 transparency varied with the humidity on two days with 

 the same amount of dust ; but the one with a wet-bulb 

 depression of 1 3° was very clear, while the other, with a 

 wet-bulb depression of only 2', was very thick. 



To show the effect of the number of particles on the 

 transparency, a number of days are selected on which the 

 humidity was the same, when it is seen that when the 

 wet-bulb was depressed /^, with 550 particles the air 

 was clear, medium clear with 814, but thick with 1900. 

 From the table a number of cases are taken illustrating 

 the dependence of the transparency of the air on the 

 number of particles in it, and on the humidity, both dust 

 and humidity tending to decrease the transparency. 

 Humidity alone seems to have no influence on the trans- 

 parency apart from the dust, but it increases the effect of 

 the dust by increasing the size of the particles. 



The modifying effect of the humidity is shown to be 

 influenced by the temperature. The same wet-bulb 

 depression which will give with a given number of 

 particles a thick air at a temperature of 60° will give a 

 clearer air if the temperature be lower. This is illustrated 

 by examples taken from the table. The increased 

 thickening effect accompanying the higher temperature 

 will be due to the increased vapour-pressure permitting 

 the dust particles to attach more moisture to themselves. 

 These remarks all refer to what takes place in what is 

 called dry air— that is, air which gives a depression of the 

 wet-bulb thermometer. 



