April 5, 1894J 



NA TURE 



545 



the Rigi. The daily maximum does not appear on all days ; 

 winds from pure directions generally prevent it, either by 

 checking the ascent of the valley air, or by the valley air 

 being pure, or by the pure valley air not being much heated by 

 the sun, and therefore having but little tendency to ri^e. The 

 daily maximum is very marked when the wind is from the 

 plains. The hour at which the rise in numbers begins and the 

 hour of maximum are very irregular. Sometimes the rise 

 begins in the morning, but sometimes it is the afternoon before 

 it puts in its appearance. The maximum number is generally 

 attained some time in the afternoon, if not checked at an 

 earlier hour by change of wind, or by clouds. The amount of 

 the daily maximum varies greatly ; sometimes it is only two 

 or three times the morning number, but it has been observed 

 as much as eight times. 



While on the Rigi the author had frequent opportunities of 

 observing the well-known tendency of Pdaius to be shrouded 

 in cloud. The clouding was frequently observed to extend 

 down the slopes far below the level of the Rigi Kulm, while 

 the Rigi kept free of cloud. It is shown that this greater 

 cloudinc'is of Pilatus is only what might be expected from the 

 nature of the surroundings. The Rigi is a true isolated moun- 

 tain, whereas Pilatus is not, though it looks quite as isolated 

 as the Rigi from many points of view. It is, however, only 

 the terminal peak of a very long wall of mountains extending 

 in a westerly direction for about twenty-five miles. As the 

 upper ridges cf this wall are from 6000 to 7000 feet high, all 

 winds from the north and west are compelled to rise when they 

 meet this barrier, and in rising condensation takes place ; 

 whereas winds from all directions can pass on all sides of the 

 Rigi, and are not compelled to rise to the same amount. It 

 is well known that the north and west win Is cause Pilatus to 

 be clouded, and these are the winds specially compelled by the 

 Pilatus range to rise and to form clouiis. 



The oliservations made at Kingairloch, in Argyllshire, are 

 then described, along with a parallel series of observations made 

 at the same hours on Ben Nevis. Diagrams are given sh iwing 

 the conditions at the two stations during July in the different 

 years. Attention is first drawn to some very abnormal dust 

 readings obtained at Kingairloch. During north-west winds 

 the number of particles is generally very low, but on the after- 

 noons of some days the numbers went high. It was found ihat 

 these abnormal readings were always accompanied by certain 

 conditions of weather : if the sky remamed completely 

 clouded all day, the numbers were always low during the 

 whole of the day ; but, on the other hand, if breaks (ormed 

 in the clouds, the number began to rise, and the in- 

 crease was very much in proportion to the amount of 

 clear sky. It is also shown that these abnormal readings 

 came far more frequently with anticyclonic than with cyclonic 

 circulation ; but as these are the conditions which bring 

 more or less cloudy skies, they do not seem to have much influ 

 ence in themselves. Tests were made to see if the abnormal 

 readings were due to local impurity, but no evidence of this 

 could be obtained. The fact that they come and go with sun- 

 shine seems to negative any such idea ; at least, if they are 

 local, they must be of a nature of which at present we know 

 nothing. It is suggested that it may be possible that sunshine 

 under certain conditions may produce some change in the con- 

 stituents of our atmosphere, which gives rise to something that 

 forms a nucleus in saturated or supersaturated air. The fact 

 that during the days of abnormally high readings the air did n')t 

 become hazed to anything like the extent indicated by the 

 number of particles, seems to suggest that these nuclei are of 

 molecular dimensions, and it is even possible they may not be 

 nuclei at all while the air is dry, but form nuclei in saturated 

 air. Nothing corresponding to these atmormal readings can be 

 discovered in the observations made at other places. In dis- 

 cussing the effects of dust, these abnormal readings have been 

 omitted, and the mean of the morning and evening figures 

 taken as the number for the day. 



The investigation of the amount of dust, and the direction of 

 the wind, shows that winds from the north-west quadrant are 

 the purest at this station, and those from the south-east quadrant 

 the most impure. All the high readings at Ben Nevis were 

 observed in south-east winds. The effect of the directi )n of the I 

 wind is shown by diagrams, the dust curves being low with | 

 north-westerly, and very high with south-easterly winds. I 



The next point discussed is the relation between the trans- 

 parency of the atmosphere and the number of dust particles. 



The observations made during the last three years, as in the 

 previous two, show that on all days when the number was small 

 the air was clear, if the wet bulb depression was over 2°. In 

 order to make the haze observations more satisfactory, it has 

 been the custom for the last three years to enter in the notes 

 the liftiit of visibility in miles at the hour the other observa- 

 tions were taken. This is done by estimating the amount of 

 haze on a mountain at a known distance, and calculating the 

 extreme limit it would be visible at in the same air. In work- 

 ing out the relation between the number of particles and the 

 transparency, it is necessary to reject all observations made 

 during rainy or doubtful weather. For reasons frequently ex- 

 plained, the observations were separated into sets, according to 

 the humidity at the time ; all the observations taken when the 

 wet-bulb depression was from z" to 4° being entered in one 

 table, all those when it was from 4' to 7' in another, and all 

 when it was 7° to 10° in a third. In the tables were entered 

 the highest, lowest, and mean numbers of particles observed, 

 while the conditions remained at all steady, and in another 

 column was entered the limit of visibility at the time. The 

 different observations were arranged in the tables in the order 

 of the mean number of particles, beginning at the top with the 

 observation with the least number, and ending at the foot with 

 the observation with the greatest number. This will be easiest 

 understood by an example. The following table represents in 

 abstract one of the tables with the Kingairloch observations for 

 1893, when the wet bulb depression was from 4' to 7'. As it is 

 unnecessary to give all the observations, only the first and last 

 are entered. 



P,,, Lowest Highest Mean ^.'T?'.',?*^ n 



Date. , •^, i_ visibility C. 



number, number. numtrer. • •, ' 



July 14 



85 ... 850 



467 ... 250 ... 117,000 J Mean. 

 - 106,000 

 July 2 ... 1600 ... 2400 ... 2000 .. 40 ... 80,000 ) 



If we look down the column headed limit of visibility in 

 the different tables, it is seen that in all of them the highest 

 limit of visibility is always associated with the least amount of 

 dust, and the least limit with the greatest amount of dust. 

 The tables show clearly that the amount of haze depends 

 directly on the number of dust particles in the air. It seemed 

 probalile that the same number of particles would produce the 

 same amount of haze, whether these panicles be distributed 

 through a long or short length of air. If this be so, then 

 the mean number of particles multiplied into the limit of 

 visibility ought to be a constant. In the tables are columns 

 headed C, in which the numbers so calculated are entered. 

 From the tables it is seen that though the values of C for the 

 different observations are not alike, yet they agree as well 

 as cou'd be expected, considering the difficulty in estimating 

 the amount of haze and the probable variations in the size 

 of the dust particles, which would influence their hazing effect. 

 The tables for the Kingairloch observations in 1893 show that 

 when the wet bulb depression was between a" and 4° the 

 value of C was 77,000 ; when the wet bulb depression was 

 from 4" to 7° C was equal to 106,000 (see table above), and 

 when the wet bulb depression was from 7' to lo'" the value was 

 141,000. 



The Kingairloch observations, when arranged in these tables, 

 show the effect of the humidity, as well as of the dust, on 

 the transparency. The value of C when the wet bulb de- 

 pression is from 2^ to 4' is only about one-half of what it is when 

 the depression is from 7° to 10". The damper air has therefore 

 nearly double the hazing effect of the drier, because C is pro- 

 portional to the number of particles required to produce a 

 complete haze, that is, a haze thick enough to shut out ail view. 

 What that number of particles really is, is obtained by multi- 

 plying the different values of C by 160,932, the number of 

 centimetres in a mile. When this is done we get the number 

 ot particles of dust per square centimetre, and of lengths of 

 fiom 10 to 250 miles required to produce complete haze in air 

 giving different wet bulb depressions. 



Number of particles required 



to produce complete haze. 



12,500,000,000 



17,100,000,000 



22,600,000,000 



Wet bulb depression. 

 2" to 4" 

 4° to f 



7° to 10° 



The above figures show the effect of the humidity very clearly. 

 Nearly double the number of particles are required to produce 



NO. 1275, '^O^' 49] 



