38 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



dust under favourable conditions. The observations were all made in a short time, and 

 under the same conditions of cloud, sunshine, and humidity. It will be seen from the 

 table that the humidity remained fairly constant during the whole time, so that the 

 thickness of the mass of air which obscured the atmosphere in the W. between 1.30 and 

 2.30 p.m., and which passed over the mountain about 2.30 p.m., and afterwards thickened 

 the air to the E., was due to an increase in dust particles and not to humidity. 



On searching into the cause of the want of uniformity in- the air on this occasion, it 

 seems possible it may have been due to the influence of the cyclone which was passing to 

 the W. of our islands at the time. On the morning of the day the Callievar observa- 

 tions were made the centre of the depression lay very near, being on the coast of Scot- 

 land immediately to the west of the place of observation, so that there would be at the 

 time a considerable difference in the directions of the winds over the immediate neigh- 

 bourhood. There would, consequently, be a considerable mixing of airs from different 

 places. 



From the tables it might be thought that the great difference in the clearness of the 

 air on the occasion of the two visits to Callievar was due to difference of humidity, as 

 the figures seem to indicate that the air was much drier during the first than during 

 the second visit. The figures in the tables, however, give no information on this point, 

 because the wet-bulb depressions for Alford, given in the table in Part I., are maximum 

 depressions for the day, calculated from the observations made at Logie Coldstone, one of 

 the Scottish Meteorological Stations situated in the same district, whereas the wet-bulb 

 depressions in the table in this paper are from observations made at the hours stated, and 

 on the mountain. The Logie Coldstone observations for 1890, however, do show that 

 the air in that year was not quite so dry as in 1889. The morning observations of the 

 day of this visit show 1 *4° less depression than the morning of the last, and the evening 

 observations 2*5° less, so that part of the greater thickness in 1890 may have been, due 

 to greater humidity. 



GarelocJihead Observations. 



February of 1891 will be remembered in Scotland as having been an unusually fine 

 February, fine though that month often is. The temperature was much above the average, 

 while the rainfall was much below it. Temperatures between 50° and 60° were frequently 

 recorded in our area, and even up to 64° was observed at more places than one. As the 

 weather continued very fine towards the end of the month, the opportunity seemed a 

 favourable one for testing the amount of dust in: the atmosphere, while we had this 

 settled and exceptional weather. For this purpose I went to Garelochhead in the end of 

 the month, and was just in time to test the air before the meteorological conditions 

 changed, and brought about a state of matters such that the March which followed the 

 fine February will long be remembered as one of the coldest experienced in Scotland. 



As will be seen from the table, the number of particles was very large and the air 

 excessively thick when it was tested on the 27th of February. The smallest number 



