ATMOSPHERE OF GREAT BRITAIN AND ON THE CONTINENT. 39 



observed this day was 7250, while other observations gave nearly 10,000. On the previous 

 visit the highest number was 2360 ; this, however, was the only occasion when it was over 

 1000. From the table it will be seen that on the 28th the conditions began to change, 

 the amount of dust having fallen considerably. The conditions continued to improve, 

 and by the 2nd of March the number of particles fell exceedingly low, and remained low 

 till the 5th, when the observations were stopped. 



On enquiring into the cause of the great amount of dust in the air in the end of February, 

 the meteorological weather charts gave the same explanation as has already been given of 

 the impure state of the air during the latter part of the last visit to Alford. For many days 

 before the 27th of February the climate of our islands was under the influence of an anti- 

 cyclone, the centre of which moved about over Europe, and gave rise to southerly winds 

 over our area. These winds brought the impure air of the Continent and England to the 

 place of observation. On the 27th the winds were still southerly and light over our area, 

 with a confused circulation, and the amount of dust was great and the hazing of the 

 atmosphere intense. But on the 28th a depression was passing to the N. of our 

 islands, and the isobars were closing in and becoming regular, with their direction E. 

 and W. Winds were, therefore, beginning to set in from the W., and, as will be seen 

 from the tables, the pure westerly winds beginning to make themselves felt, the 

 number of particles fell from near 10,000 to 1750. On the 1st of March the iso- 

 bars closed still further, and were situated more due E. and W. The conditions con- 

 tinued to strengthen next day, the 2nd, by which time the isobars had become very close, 

 and the winds had veered a slight amount to N. The result of the change is seen in 

 the table ; the dust went down to 51 per c.c, or about ■ 2 -^ Ty of what it was three days 

 before. During the 3rd, 4th, and 5th, the isobars kept much the same position as on the 

 2nd, and the amount of dust continued very low. It may be stated that the isobars 

 opened a little on the 3rd and 4th, but came close again on the 5th ; and it will be 

 noticed that the dust increased a little on the 3rd and 4th, but decreased again on the 

 5th. The interpretation of this may be, that closely-situated isobars indicate great 

 general circulation and consequent reduction of local impurity, though it may be noticed 

 that the local wind was not so high on the 5th as on the two previous days. 



Dust and Isobars. 



In Part I. it is shown that the amount of dust depends greatly on the force of the 

 wind, and also on its direction. In working out the results of these last observations the 

 effect of the direction comes out in a very marked way. Although all increase in wind is 

 accompanied by a decrease in dust, yet it would appear that certain directions of wind 

 have a much greater purifying influence than others, i.e., winds from certain directions 

 are purer than those from others. The directions of the purest winds are not the same at 

 all stations ; the conditions of the areas surrounding the stations determining the purest 

 directions. In Switzerland, the southerly winds are pure, while northerly ones are 



