214 



KNOWLEDGE. 



[November 1, 1808. 



Brooks's comet (V., 1889), whilst it was in the immediate 

 vicinity of Jupiter. This vrill include a discussion of the 

 questions of the probability of the disruption of the comet, 

 and of the possibility that a portion of it was drawn 

 permanentl}' into the .Jovian system, becoming a species 

 of satellite of that planet. 



DUST AND ATMOSPHERIC PHENOMENA. 



By Dr. J. G. McPheksoxV, F.R.S.E. 

 (Lecturer on Meteorology in the I'nieersity of St. Andreu-n). 



AT a recent meeting of the Eoyal Society of Edin- 

 burgh, Mr. John Aitken read a most interesting 

 and important paper on " Dust and Atmospheric 

 Phenomena." It was a continuation of the 

 investigations made in 1880, which I abridged 

 in these columns in October, 1890. During 1890 he went 

 over the same ground under different atmospheric con- 

 ditions. He first visited Hyt^res, where by his famous 

 instrument he counted 250,000 dust-particles in a cubic 

 inch of the air, whereas on the former visit he counted only 

 12,000. He observed, however, that where there was 

 least dust the air was very clear, whereas with the 

 maximum of dust there was a very thick haze. 



At Mentone, the number of dust-particles in the cubic 

 inch was 13,000 when the wind was blowing from the 

 mountains, but increased to 130,000 when the wind was 

 blowing from the populous to-mi, where the air was more 

 polluted. At Bellagio the result was similar. With the 

 smaller number the air was clear and brilliant, with 

 scarcely any perceptible haze on the hills ; but with the 

 larger number there was a thick haze, the distant hills 

 being quite invisible. After a series of observations there, 

 with careful examination of the temperature by wet bulb 

 as well as dry bulb thermometer, he came to the conclusion 

 that the increase in the number of dust-particles accom- 

 panied by constant humidity is accompanied by a decrease 

 in transparency ; and that the increase in humidity is also 

 accompanied by decrease in transparency if the number of 

 dust-particles remains constant. 



When Mr. Aitken first visited the Eigi Kulm, in 

 Switzerland, the air was remarkably clear and brilliant, 

 and the number of dust-particles per cubic inch never 

 exceeded 33,000. On his second visit, in May, 1890, he 

 counted no less than 160,000— about five times the number ; 

 this was accompanied by a thick haze, which rendered the 

 lower Alps scarcely visible. The thickness was not due to 

 humidity, for the wet bulb was depressed 10°, therefore 

 the air was very dry ; but it was due to the number of 

 dust-particles suspended in the atmosphere. On looking 

 down from the mountain to the valleys, the air was, some 

 time before sunset, so thick that the lower slopes of Pilatus 

 were scarcely to be detected. This was due to a thick 

 haze between the observer and the mountains, as if a veil 

 were hung between him and the distant scenery. The 

 upper limit of the haze was well defined, and though the 

 sky was cloudless, the sun looked like a harvest moon, and 

 required no eagle's eye to keep fixed on it. 



Next day there was a violent thunderstorm, which gave 

 an opportunity of making observations at its different 

 stages. At noon, when it was clear, the number of 

 particles to the cubic inch was 66,000. At 6 p.m. the 

 storm commenced, and 60,000 to the inch were registered ; 

 but in the middle of the storm he counted only 13,000. 

 There was a heavy fall of hail at this time, and Mr. 

 Aitken accounts for the diminution of dust-particles by 

 the downrush of purer upper air, which displaced the 

 contaminated lower air. For the year before, ho observed 



the same change during a heavy fall of rain on the Eiffel 

 Tower, when the number per cubic inch was as low as 3000. 



He next showed, from a reference to the general circula- 

 tion of the wind over Switzerland during the periods of 

 observation, that when the number of particles was small 

 during his first visit the general air circulation was from 

 the Alps ; and, on the second visit, when the number was 

 great, the movement of the air was from the inhabited 

 parts. He also found that on the day of the storm the 

 diminution of the number of particles was concurrent with 

 a change in the direction of circulation, which brought 

 pure air from the Alpine region, and it was when the pure 

 air and the foul air began to mix that the storm broke. 



But the most remarkable set of observations was made 

 on the Lake of Lucerne. In the course of an hour there 

 was an exceptional diminution in the number, which 

 made him think that something had gone wrong with his 

 instrument. At 3 p.m he counted 171,000 in a cubic inch, 

 at 3.45 the number was reduced to 28,000. During the 

 interval the thermometer had risen from 71° to 75", and 

 the wet bulb depression varied from 11° to 19°. On 

 looking about, he found that the direction of the wind had 

 changed, bringing down the purer upper air to the place of 

 observation. The rapid change in the direction of the lower 

 current of air was caused by a south-west wind striking 

 the face of the mountain, which was there nearly vertical. 

 The bending downwards of the trees by the strong wind 

 showed that it was coming from the upper air. At 4 p.m. 

 the thermometer fell to 68°, and the wet bull) depression 

 was 10° ; he counted 204,000 particles in the cubic inch 

 — a most remarkable result — but the old conditions were 

 restored by the reversal of the wind. 



Pieturning to Scotland, Mr. Aitken continued his 

 observations at Ben Nevis and at Kingairloch, opposite 

 Appin, Mr. Eankin using the instrument at the top of the 

 mountain. These observations showed in general that on 

 the Ben southerly, south-easterly, and easterly winds were 

 more impregnated with dust-particles, sometimes containing 

 133,000 per cubic inch. Northerly winds brought pure 

 air. The observatinns at Kingairloch showed a certain 

 parallelism to those on the summit of the mountain. With 

 a north-westerly wind the particles reached the low number 

 of 300 per cubic incli, the lowest recorded at any low-level 

 station. 



The general deductions made by Mr. Aitken from his 

 numerous observations during the two years are that air 

 coming from inhabited districts is always impure ; dust is 

 carried by the wind to enormous distances ; dust rises to 

 the tops of mountains during the day ; with much dust 

 there is much haze ; high humidity causes great thickness 

 of the atmosphere if accompanied by a great amount of 

 dust, whereas there is no evidence that humidity alone has 

 any eft'ect in producing thickness ; there is generally a 

 high amount of dust with high temperature, and a low 

 amount of dust with low temperature, and a high amount 

 of dust reduces the transparency of the air. 



CURIOUS COCOONS.-I. 



By E. A. Butler. 



THOSE insects which, just before assuming their 

 final form, pass into a limbless, inactive condition, 

 or, in other words, become a chrysalis and under- 

 go a complete metamorphosis, evidently need some 

 special protection during this period of rest, when 

 they are unable to look after their own interests. And 

 the need is intensified by the fact that the period of pupa- 

 hood often lasts throughout the winter, so that the insect 

 spends a larger proportion of its life in this helpless and 



