ATMOSPHERE IN GREAT BRITAIN AND ON THE CONTINENT. 21 



previous mornings during this visit : the air still had the same thickness, the amount 

 of dust was high for the morning, and there was little change in the humidity. Before 

 8 a.m., when the first observations were being made, small clouds occasionally formed on 

 the S.E. face of the hill and passed over the place of observation. While in these 

 clouds, the temperature fell and the humidity increased ; the temperature being 46° 

 and the wet-bulb depression about 1°, which was 2° colder than the surrounding air and 

 3 '5° less depression of the wet bulb. 



By 10 a.m. the clouds ceased passing over the mountain, but were still forming lower 

 down. As the day advanced, clouds began to form on the Alps to the S., and 

 these seemed to have their origin in the air coming from the same sources as on the 

 previous day ; one mass formed to the S.W. and moved northwards, while another 

 mass came streaming over the Alps from the S. and formed a mass in the S.E. 

 and spread northwards, while over the Rigi the sky was clear. Both on this occasion 

 and on the previous day the formation of the mass of cloud in the E. was particularly 

 interesting ; on both occasions it formed in a cloudless sky. It seemed to be caused by 

 hot moist air driven up the southern slopes of the Alps by the wind, and appeared to 

 come streaming northwards through some opening in the mountains. At first the 

 current formed only a long thin cloud, but as the day advanced the current strengthened 

 and the little stream gradually grew and expanded as it moved northwards, and rose in 

 the air ; and by the time the day was well advanced, it had grown to a mighty mass 

 of thunder-cloud, thousands of feet thick, which completely filled the eastern sky. It 

 would be difficult to imagine anything grander than this billowy mass of thunder-cloud 

 as it moved northwards shining brightly in the sunlight. 



Between 4 and 5 p.m. the clouds to the S.W. had increased considerably. They 

 were forming at an elevation very little above the top of the Rigi. About 5 p.m. thunder 

 was heard to proceed from this mass. After a time the active area seemed to move east- 

 wards, i.e., to the S. of the Rigi. It then went to the S.E., and finally it came 

 directly overhead. As this seemed a favourable opportunity for testing the effect of a 

 thunderstorm on the amount of dust, observations were at once begun, and as many 

 tests taken as possible while the storm lasted. At 6 p.m. there was a marked increase 

 in the violence of the storm. Large hailstones fell thickly and with great force while 

 the tests were being, made, which made working extremely difficult. So near was the 

 storm at this time that the thunder followed close on the flash, and on one occasion no 

 perceptible interval was noticed between a very brilliant flash and the deafening crash 

 which accompanied it. 



It has often been contended that thunderstorms cause the " turning " of milk and other 

 putrefactive actions by some effect they have in bringing about the deposition of living 

 organisms floating in the atmosphere. If thunderstorms really have this effect, then we 

 would, expect that they would cause the fine dust in the atmosphere to settle also. I 

 had long been desirous of testing this point, to see if thunderstorms had really any 

 effect on the amount of dust in, the atmosphere, but as yet had but few opportunities, as 



