ATMOSPHERE OF GREAT BRITAIN AND ON THE CONTINENT. 



43 



It may, of course, be contended that we are here stretching a point to support the 

 theory that dust checks terrestrial radiation and consequent cooling, as the only 

 evidence produced to show that the air was pure during the cold period is the direction 

 of the air circulation after the country was covered with pure air. However, as the 

 circulation was purely local, it seems probable the increase in impurity would be slow 

 compared with the sudden increase of dust which we have seen takes place with a change 

 of circulation. 



After writing the above, it appeared to me that the point was one of some importance, 

 and worth following up. I therefore trespassed so far on the kindness of the Scottish 

 Meteorological Society as to ask for information regarding the state of the air as observed 

 on Ben Nevis during March. Mr Rankin, the observer at that date, has kindly written me 

 as follows : — " The observations of March dust are very interesting. There are two well- 

 marked periods of purity, 1st to 12th, beginning and ending sharply, again 25th to 29th." 

 During the first period the numbers were excessively low ; a not unusual reading was 7 

 per c.c, and sometimes it was much lower. From the extract from Mr Rankin's letter it 

 will be seen there was the same rapid change on the Ben from impure to pure air in the 

 beginning of March as was observed at low level. It will be also seen that during the 

 period of excessively low temperature at low level, the upper air was remarkably free from 

 dust. In this case the evidence in support of the relation between the dust and tempera- 

 ture is very strong, as the air was exceptionally cold during this exceptionally pure 

 period. 



Table of the Number of Dust Particles in the Atmosphere. 



Place. 



Date. 



Hoxir. 



Number of 



Particles 



per c.c. 



Wind. 



Tempera- 

 ture. 



Humidity. 



State of 

 the Air. 



Remarks. 





1890. 

















H yep.es . . 



Mar. 31 

 April 



3.30 p.m. 



725 



S.W. 3 



56 



9-5 



Very clear 



Observed on Fenouillet. 



)> 



1 



3 P.M. 



15,250 



E. 2 



59- 



5 



Very thick 



» » 



Cannes . . 



5 



5 P.M. 



2,850 



N. 0:2 



58 



6 



Haze 



Observed on La Croix des 

 Gardes. 



>) 



7 



10.45 a.m. 



1,275 



N. 0'2 



60 



8 



Medium 



» » 



Mentone . . 



11 



4 P.M. 



14,000 



W. 3 



56 



)> 



Thick haze 



Observed on hill 800 feet 

 high. 



)> 



12 



4.30 p.m. 



875 



N. 1 



48 



3-5 



Clear 



Observed on hill. Snow on 

 hills to half-way down. 



i) 



14 



3 P.M. 



810 



S.E. ? 1 



50 



4 



Raining 



Raining all the 13th, 



!> 



16 



3 P.M. 



900 



E. ? 0'5 



54-5 



2-5 



Medium 



Observed on hill. 



)> 



19 



4 P.M. 



26,000 



S.E. 0'5 



58 



3-5 



Thick 



Observed on hill. Number 

 varied greatly. 



Bellagio . . 



23 



3 P.M. 



6,300 



S. 2 



65 



8 



Thickish 

 haze 



Limit of visibility 15 miles S. 



>» 



24 



10 A.M. 



1,225 



N. 4 



58-5 



16 



Clear 









11.30 A.M. 



5,300 



N. 1 



59 



16 



Hazy 



Air much thicker than at 



10 A.M. 



