CORRESPONDING SOCIETIES. 435 



Referring to Table II, we find the higheet mean monthly deposit recorded was 

 35 tons per square kilometre, or about 90 English tons per square mile at Rochdale 

 during the winter of 1917-18. The summer of the same year gave 34-62 tons per 

 square kilometre at Rochdale. 



Oldham during tlie winter of 1914-15 gave a mean monthly deposit of 34-7 tons 

 per square kilometre. 



The lowest deposit recorded wae at Malvern and amounted to 1-69 tons per 

 square kilometre during the summer of 1915. The Malvern gauge is situated in 

 the open country at Malvern Wells, and may be taken as indicating what we 

 expect our country air to be like if luicontaminated artificially by the smoke and 

 dust of cities. Doubtless Malvern receives some pollution from the cities sur- 

 rounding, 60 that, strictly epeaking, the air is not absolutely pure, but for our 

 present purpose we may regard it as representing country air. 



During the winter of 1917-18, when Rochdale gave 35 tons per square kilometre, 

 St. Helens gave 21 and Malvern 2-12; thus the Rochdale deposit was over sixteen 

 times that of Malvern Wells, whilst St. Helens deposit was about ten times. 



When we examine the table for evidence of improvement or otherwise we get 

 the following results, excluding the stations with only two years' record : — 



The Coatbridge figures show clear evidence of increasing impurity, as the rise 

 in the amount of deposit during the five winters recorded is very regular and 

 marked. 



Glasgow shows a considerable drop in tlie amount of deposit in 1918-19 when 

 compared with previous years, and we may say on the whole there is a tendency 

 there towards improvement. 



Greenock, observing for three years only, showed a steady deterioration in 

 its air. 



Leith, also for three years, indicated a steady deterioration. 



The other stations give no clear indication of improvement or deterioration 

 in the air over them. There are, of course, variations from year to year, but it 

 is impossible to say at this stage if a change in the 1918-19 figures represents 

 the beginning of a steady move in either direction. 



It would seem from these figures that the shortage of coal and the rationing 

 order has not produced any very marked effect in purifying the air. 



To get an idea of the relation which the different constituents of the deposited 

 impurity bear to each other, curves have been prepared for several years, giving 

 the average deposit of each element of pollution for each month for a group of 

 stations varying in number from year to year. These have been published in 

 the Committee's Reports.' 



The scales for the different curves are not all the same ; but a good idea of the 

 relationship of the different elements can be obtained from these curves. It 

 will be seen that the soluble impurities tend to vary together and have a distinct 

 relation to the amount of rainfall. In these curves the ordinates represent tona 

 per square kilometre except in the case of the rainfall, which is given in deci- 

 metres. These are plotted above the months, and so the seasonal variation is 

 brought out also. In all these curves it will be observed that the insoluble 

 ash is the only one showing a tendency towards a summer maximum ; for example, 

 in the year ending March 31, 1919, insoluble ash made its highest deposit in May 

 and its loweet in August or September. 



In dealing with the impurities deposited from the air we have to remember 

 that this does not tell us the whole story. There are suspended particles of 

 such small dimensions that they practically do not deposit and are carried ove^ 

 very wide areas by the winds before suitable conditions for deposit are attained. 

 The rneasurement of deposited matter does, however, tell us what amount of 

 impurity is received from the air on buildings, plants, agricultural land, etc. The 

 more finely divided and permanently suspended impurity is, however, of great 

 importance, and the investigation cannot be regarded as complete unless this 

 be also taken into account. 



First^ to Fifth Annual Reports, Advisory Committee on ' Atmospheric 

 Pollution,' obtainable from the Honorary Secretary, 47 Victoria Street, Lrondnn, 

 S.W.I. 



