ATMOSPHERE IN GREAT BRITAIN AND ON THE CONTINENT. 35 



have to be made on the hazing of high mountains, they have, therefore, always to be 

 made on a good deal of upper air, which may vary greatly from the air tested at low 

 level. Further, there is a fundamental error in this method of estimating haze which 

 was not observed till after the observations had been made. As we have said, when the 

 air is clear the haze on some distant mountain has to be estimated. And in making 

 this estimate one compares the whiteness of the haze on the mountain with the whiteness 

 or brightness of the background, which in this case is the sky. Now, it is evident that 

 if the sky be full of white clouds, and. the background bright, it will require far more 

 haze to make the mountain invisible than if the background were darker. Again, 

 suppose there are no clouds and the background is only haze, and suppose the con- 

 dition to be such that the mountain seems half lost in haze. If we were now to double 

 that haze, the mountain would not become invisible, because whilst the air between us 

 and the mountain has been thickened, the brilliancy of the background, against which we 

 see^ the mountain, has also been; increased. If the brilliancy of the background were 

 doubled, the mountain would still be only half hazed ; but, of course, double hazing 

 will give less than double brilliancy to the background. It seems to be for these 

 reasons that the estimates for clearness in dry air do not show the hazing effect of the 

 number of particles in the same way as when the air is damper, and the estimates of 

 haze are made on lower and nearer mountains, and, therefore, more in the air: tested. 

 The estimates of haze when the air is dry being of no value, they have not been 

 entered in the table. It is evident from these remarks that a , more accurate method 

 of measuring haze is required. 



In the Appendix to Part I., when discussing some observations made during a gale 

 of wind, it is pointed out that high winds will probably have the effect of making the 

 air look thick for the number of particles and the humidity. This conclusion is con- 

 firmed by the observations in the table given with this paper. In all the observations 

 taken while, or immediately after, the wind was high, the transparency was very low for 

 the number of particles and the humidity. As will be seen from the table, on the 9th, 

 22ndj and 24th July, the wind was high and the transparency very low for the other con- 

 ditions. On some other days the transparency was also low for the dust and humidity, 

 but on these days the thickening was due to passing showers. The hazing effect of high 

 winds would seem to be due to the air carrying large particles, such as dust from roads, 

 &c, and also to the wind mixing impure lower air with the upper. Part of the hazing 

 may also be due to the unequal densities of the -mixed airs. 



Alford. 



At this station both* purer and less pure air was observed in 1890 than in 1889. As 

 will be seen from the figures, in the tables, the purest observed in 1889 had about 500 

 particles per c.c, whereas in 1890 under 200 per c.c. was observed on three occasions. 

 During last visit the maximum; was 5700, while on this visit it was 6800 per c.c. _j 



