652 



MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



;i greater limit of visibility than 250 miles. This, however, may be explained by the fact 

 that when the air is very clear it is very difficult to estimate the amount of haze, and all 

 very clear days have simply been put down at 250 miles, so that, though they had been 

 clearer than the figures indicate, they would still have been entered as only 250 miles. 

 The observations, however, on the 22nd did not show the air to have its maximum clear- 

 ness, though the number of particles was very low. The reason for this is that, during 

 the period of the observations on that day, it was raining on the hills, and as the under- 

 side of the clouds was ragged, it looked as if raining overhead, and the drops seemed to 

 evaporate in the lower air before coming to the ground. The upper air would thus be 

 unduly damp, and therefore thicker than it would have been under ordinary conditions. 

 The only other number in this table calling for remark is the highest number of particles 

 on the 23rd. It was too high, owing to the number not having fallen from the abnormal 

 sunshine maximum. In Table XIII. the mean number of particles is too high for the 

 visibility on the 17th, 13th, 25th, 10th and 15th. This was owing to the evening- 

 numbers on the first three of these days not having fallen from the sunshine maximum 

 to near their lowest number, or to what they were the next morning. The high reading 



Table XII. — Showing the Relation between the Number of Dust Particles and the Transparency 

 of the Atmosphere at Kingairloch when the Wet-Bulb Depression was from 4° to 7°. 



Date. 



Lowest 

 Number. 



Highest 

 Number. 



Mean 

 Number. 



Limit of 



Visibility in 



Miles. 



C. 



1892. 

 18th July 

 22nd „ 

 20th „ 

 15th „ 

 23rd „ 

 14th „ 



123 

 132 



308 



750 



290 



1250 



129 



245 

 675 

 975 



1800? 



3575 



126 

 188 

 491 

 862 



1045? 



2412 



250 



200? 

 200 

 130 

 130 

 50 



(31,500) 

 (37,600) 



98,2001 

 112,060 [ Mean, 

 135,850 f 116,677 

 120,600 J 



Table XIII. — Shoiving the Relation between the Number of Dust Particles and the Transparency 

 of the Atmosphere at Kingairloch when the Wet-Bulb Depression was 7° and over. 



Date. 



Lowest 

 Number. 



Highest 

 Number. 



Mean 



Number. 



Limit of 



Visibility in 



Miles. 



C. 



1892. 

 16th July 

 17th „ 

 13th „ 

 12th „ 

 25th „ 

 10th „ 

 15th „ 



161 



252 



900? 



385 



511 



925 



162 



359 



1225 ? 

 1025 ? 

 1125 

 1 250 ? 

 1500? 

 3000 ? 



260 



738? 



962? 



755 



880? 



1212? 



1731? 



250 



250 

 250 

 200 

 200 

 150 

 130 



65,000 " 

 184,500 

 240,500 

 151,000 

 176,000 

 181,800 

 225,030 



, Mean, 

 r 174,832 



