<j-48 Ml! JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



The transparency observations for 1892 do not call for any special remarks. On 

 most of the days the air was pure, and whenever it was at all dry it was clear or very 

 dear. There are no well-marked periods of dusty air during these observations either 

 at high or low level. Most of the high readings at low level were the abnormal readings 

 in N.W. winds and sunshine, while the high readings on the Ben were generally at 

 night due to impure air brought up by south-easterly winds. 



An examination of Table III. and Diagram III. shows that during the 1893 observa- 

 tions there were well-marked periods of low and high numbers. As the observations 

 made during this period are more characteristic of the effects of dust on transparency 

 than those of the previous years, we shall enter somewhat fully into an analysis of them. 

 At the outset we may as well remark that, for reasons already given, we must omit in 

 this analysis all the observations made when there was sunshine and N.W. winds, and 

 on these days we shall take the morning and evening readings as representing the number 

 of dust particles for the day. 



In previous communications, when discussing the connection between the number of 

 dust particles and the amount of haze, attention has been called to the necessity of either 

 comparing only days when the humidities were the same or of making an allowance for 

 the differences in humidities. What this allowance is has been roughly indicated in the 

 paper already referred to, on the hazing effect of atmospheric dust. In the present inves- 

 tigation, however, as in the previous ones, it will be better to compare only days when 

 the humidity was the same, and then see if the observations made with a dust-counter 

 confirm the conclusion regarding the effect of humidity arrived at, in the paper referred 

 to, from observations made without a dust-counter. 



To show the hazing effect of the dust in the 1893 observations, the first thing to be 

 done was to separate the suitable observations in Table III. and rearrange them in tables 

 according to the wet-bulb depression at the time, so that the humidities of a]l the obser- 

 vations in each table should be nearly the same. In one table, as in the previous 

 investigation, were entered all the observations taken when the wet-bulb depression 

 was 2° and under ; in another, all the observations when it was between 2° and 4°; 

 in a third, all the observations when it was between 4° and 7°; and in the fourth, all the 

 observations when it was 7° and over that amount. As there was only one observa- 

 tion in the first table, that is with a wet-bulb depression of 2°, it was entered into the 

 next table. The principal reason for there being so few observations with very high 

 humidity in the tables is, that the wet bulb seldom remains for any length of time at 

 or below 2°. No observations taken while it was raining, or while the weather was 

 -liMwery, are available for our present purpose, as humidity and transparency observa- 

 tions under these conditions are unreliable. 



In selecting the observations for our present purpose, single observations were not 

 generally used, as it was thought a more correct result would be obtained by using periods 

 <luring which the conditions remained fairly settled, that is while the number of particles, 

 the humidity, and the transparency were fairly steady. Sometimes these periods were 



