(MO MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



vations are connected by straight lines. The low-level observations are represented by 

 large dots connected with thick lines, while the Ben Nevis observations are represented 

 by smaller dots and connected with finer lines. 



As in the diagram given in Part II., the winds in Diagrams I., II. and III. are indicated 

 by three series of arrows. The arrows at the top of the diagrams show the direction and 

 force of the wind on Ben Nevis at the hour the dust observations were made, and the 

 arrows at the bottom represent the winds at Kingairloch. The intermediate series of 

 arrows represents the general air circulation over the British Isles. The direction of these 

 arrow r s has been taken from the weather charts of the Meteorological Office, London, 

 kindly supplied to me by Mr Scott. It will be noticed that in this intermediate series 

 there are frequently two or three arrows placed together, while sometimes there is only 

 one. The meaning of this is, that if the air circulation over our islands is regular, and 

 the wind is from one direction at all places, then one arrow represents the conditions. 

 But when the circulation is mixed, the wind blowing one way at one place and another 

 way at another, then two or more arrows are required to represent the conditions. The 

 letters C and A placed alongside these arrows indicate whether the circulation was 

 cyclonic or anticyclonic. 



It has already been pointed out in Part II. that, when the isobars are close and regu- 

 lar, and lie in certain directions, so that the winds are strong and blow from certain 

 quarters over all our area, the number of dust particles is small ; and that when the 

 isobars become open and irregular, the winds becoming slight and irregular, the number 

 of particles may become great even when the wind blows from a pure direction, — the 

 reason for this being that, though we are testing air coming from a pure direction, we 

 may yet be testing polluted air, which has come from inhabited areas to the place of 

 observation, and has not been swept away for want of a regular circulation. By examining 

 the intermediate series of arrows in the diagrams we can at once see whether the circula- 

 tion over our area is regular or confused. This enables us to explain some of the dust 

 observations which at first sight may appear rather unintelligible. 



At pages 29 and 30, Part II., attention has been called to some abnormal dust observa- 

 tions made in 1890. It is shown that, though the winds from W. to N. bring 

 exceptionally pure air to Kingairloch, yet on six days when the wind was north- 

 westerly the number of particles went high at some time of the day, though always low 

 in the morning. These exceptional readings are difficult to understand, as there is 

 nothing quite corresponding to them with the wind from other directions. Let us now 

 turn to the records of the last three years (Tables I. , II. and III., Diagrams I., II. and III.) 

 and see how far they confirm these abnormal readings. This will be easier studied by 

 means of the diagrams than by the tables. On looking at Diagram I. for 1891 it will 

 be seen that when the wind was N.AV. the numbers went very high on the 9th, 23rd, 

 27th, 30th and 31st July and 2nd August ; in 1892 the numbers went high with 

 X.W. winds on the 20th, 23rd and 25th ; and in 1893 they went high on the 25th and 

 26th June and on the 3rd, 13th, 14th, 15th and 17th Jul}-. The most remarkable of 



