ATMOSPHERE OF GREAT BRITAIN AND ON THE CONTINENT. 647 



transparency. It has been pointed out that the amount of haze increases with the 

 number of particles present ; it has also been shown that the hazing effect of the dust is 

 affected by the humidity of the air at the time. Further, in a paper on the hazing 

 effects of atmospheric dust* it is shown that, for a given number of dust particles, the 

 transparency of the air, or absence of haze, is, roughly speaking, directly proportional to 

 the wet-bulb depression. The effects of the dust and the humidity are also evident in 

 the observations for 1891, 1892 and 1893. 



Looking at Table I. and Diagram I. for 1891, it will be noticed that during the period 

 of these observations the number of particles on most days was small, and sometimes 

 very small. And if we look down the column headed "State of the air" we shall see that 

 the air during these pure periods was generally clear, and often very clear, when the 

 wet-bulb depression was 2° or more. It may be mentioned that when the wet-bulb 

 depression is less than 2° it is unreliable, as it is generally obtained either immediately 

 before rain, when the atmosphere is thickening to rain, or immediately after rain, when 

 the ground and trees are wet, — humidity observations under these conditions are of no 

 value. It should also be mentioned that all observations made while it is raining must 

 be rejected when considering the relation between the dust and transparency. The only 

 days in the 1891 observations when the number of particles was high were the 18th and 

 19th; but as on these two days the air was very dry, the wet-bulb depression being 

 generally over 8°, the amount of haze was never great, and the transparency varied from 

 thichish to clear. 



In Tables II. and III. for the years 1892 and 1893 will be found, in the column 

 headed " Remarks, " a series of numbers placed between brackets. These numbers 

 represent the limit of visibility of the air at the time, calculated from observations made 

 on the amount of haze between the observer and mountains at known distances. As 

 explained in previous papers, this is done by estimating the amount to which the moun- 

 tain appears hazed. Say it looks half-hazed, and it is 20 miles distant, then that gives a 

 limit of visibility of 40 miles. Beyond that distance no mountain could be seen in that 

 air. These estimates must necessarily be only rough approximations to the transparency, 

 because at this station they can only be taken in a S.E. direction, mountains closing 

 in the view at all other points. This will cause the morning observations to appear more 

 hazed than the afternoon ones, with the same amount of haze present, owing to the 

 morning estimates being made in the direction of the sun, while the afternoon ones are 

 made in a direction at right angles to it. Further, these estimates have to be made 

 through much of the upper air to the upper slopes of the mountains, and we do not 

 know enough about the condition of the air at these elevations. Even the Ben Nevis 

 observations do not supply all that we require, the mountain being so often in cloud, and 

 the air of which is frequently of a different character from that below the clouds, through 

 which the estimates have to be made. 



* " On some Observations made without a Dust-Counter on the Hazing Effect of Atmospheric Dust." Proc. Boy. Soc, 

 Edin., vol. xx. p. 76. 



