January 28, 1892^ 



NATURE 



299 



authorities on which Dupuis founds his statement, but I 

 have no doubt that it is amply justified, for the reason 

 that doubtless all the inscriptions in the deepest tombs 

 were made by means of reflected sunlight, for in all 

 freshly-opened tombs there are no traces whatever of any 

 kind of combustion having taken place even in the inner- 

 most recesses. So strikingly evident is this that my 

 friend M. Bouriant, while we were discussing this matter 

 at Thebes, laughingly suggested the possibility that the 

 electric light was known to the ancient Egyptians. 



With a system of fixed mirrors inside the galleries, 

 whatever their length, and a movable mirror outside to 

 follow the course of an Egyptian sun and reflect its 

 beams inside, it would be possible to keep up a constant 

 illumination in any part of the galleries, however remote. 



Dupuis quotes another statement that the greatest pre- 

 cautions were taken that the first rays of sunlight should 

 enter a temple (of course he means a solar temple). 



J. Norman Lockyer. 

 ( To be continued) 



ON THE NUMBER OF DUST PARTICLES IN 

 THE ATMOSPHERE OF VARIOUS PLACES 

 IN GREAT BRITAIN AND ON THE CON- 

 TINENT, WITH REMARKS ON THE 

 RELATION BETWEEN THE AMOUNT OF 

 DUST AND METEOROLOGICAL PHENO- 

 MENA.— Part II} 



'T'HIS paper contains the results of the observations 

 -*■ made on the dust of the atmosphere at various 

 places in 1890. These observations were made by the 

 author at the same stations, and about the same dates, as 

 those made in 1889, and given in Part I. of this subject, 

 read before the Society on February 3, 1890. 



At Hy^res, in 1890, the highest number of dust particles 

 observed was 15,000 per c.c, with a wet bulb depression 

 of 5°, the atmosphere at the time being very thick. The 

 lowest was 725 per c.c, with a wet bulb depression of 9°-5, 

 when the air was very clear. 



At Cannes very few observations were made on this 

 occasion. The numbers varied from 1275 to 2850 per c.c. 

 The wind during the time was always northerly. 



At Mentone the numbers varied from 26,000 per c.c. 

 when the wind was from the town, to about 900 per c.c. 

 when it was from the mountains, with a wet bulb depres- 

 sion on both occasions of 3^° ; the air was clear with the 

 lower number and thick with the higher. 



At Bellagio, when the wind was southerly — that is, from 

 the inhabited districts— the number of particles was great, 

 on one occasion as high as 20,000 per c.c. But when the 

 wind blew from the north— that is, from the direction of 

 the Alps — the number fell as low as 600 per cc. on one 

 occasion. ^With the low numbers the air was clear, 

 whereas with the high numbers there was always a good 

 deal of haze, though the air was dry. 



All the observations made at Baveno were made while 

 the wind blew from the inhabited areas, and the air was 

 never clear, although on some days it was very dry. 

 The highest number observed at this station was 16,000 

 per c.c, and the lowest 2000 per c.c. 



The observations made at the Rigi Kulm from May 

 15 to 20 are then discussed. There was a marked 

 difference in the appearance of the air on this occasion 

 compared with what was seen on the first visit. During 

 the previous visit the weather was generally fine, and the 

 air had that crisp clearness which gives the hard outline 

 and crude colouring one generally associates with Swiss 

 scenery ; whereas on the second visit the air was remark- 

 ably thick and heavy. 



' Abstract of a Paper read before the Royal Society of Edinburgh on 

 January 4, by John Aitken, F. R.S. Communicated by permission of the 

 Council of the Society. 



NO. I 161, VOL. 45] 



The highest number observed on the first visit to the 

 Rigi was not much over 2000 per c.c, while the number 

 was as high as 10,000 on the second. The same relative 

 condition of impurity existed at the low level also. On the 

 first occasion the number at the level of the lake varied 

 from 600 to 3000 per c.c, whereas on the second visit 

 they varied from 1700 to 13,000 per c.c. Roughly 

 speaking, there was about four times the amount of dust 

 in 1890 there was in 1889, and the air was about four 

 times as thick. 



On the way up the Rigi the air was tested at the level 

 of the lake and found to have about 11,000 particles per 

 c.c There was a very thick haze at the time, through 

 which the mountains loomed darkly. This thickness 

 was evidently not due to humidity, as the wet bulb showed 

 a depression of 10 degrees. This very thick haze was 

 therefore due to fairly dry particles of dust. On arriving 

 at the top of the mountain in the afternoon, the air was 

 tested and found to have slightly over 4000 particles 

 per c.c 



During the first four days of the second visit to the 

 Rigi Kulm the air was very thick and the number of 

 particles great. The much greater thickness of the air 

 on the occasion of the second visit was evidently due 

 to dust, as the humidity on both occasions was about the 

 same. The air on the days when the number of particles 

 was great was very different from anything previously 

 observed at this station. On the first visit the air was 

 clear and bright, with only a thin haze between the 

 observer and the distant mountains ; whereas in 1890 a 

 dense haze hung in the atmosphere, so thick that towards 

 sunset the lower slopes of Pilatus could scarcely be dis- 

 tinguished. It looked as if a veil had been hung up 

 between the observer and the distant scenery. Some 

 time before sunset this hazy veil became coloured by 

 the rays of the setting sun. Its upper limit was well 

 defined in the eastern sky, at an elevation considerably 

 above the highest of the Alps. At sunset this dusty 

 impurity became still more apparent as the earth's 

 shadow crept up its lower edge. Though the sky was 

 cloudless, so dull was the setting sun, that it looked more 

 like a harvest moon than the orb of day. So feeble were 

 its rays after penetrating the thick haze, that they could 

 produce no direct red light on the mountains, while much 

 diffused light was reflected by the dust-laden air. 



During this second visit there was an opportunity of 

 testing the supposed influence of thunderstorms in de- 

 positing the dust in the atmosphere. On one of the days 

 of this visit a violent thunderstorm raged to the east, 

 south, and west during most of the afternoon, and in the 

 evening it came over the Rigi Kulm. So near was the 

 storm that the flash and crash of the thunder seemed 

 simultaneous. The tests were therefore made in the very 

 air in which the lightning discharges were taking place. 

 During the day, and before the storm approached, there 

 were nearly 4000 particles per c.c. in the air. At 6 p.m., 

 when the storm was near, the number fell to 3000 ; and at 

 7.10 p.m., when the storm was nearly over, the number 

 was as low as 725 per c.c. These figures seem to support 

 the supposition that thunderstorms purify the air ; and 

 if anyone who was a believer in the purifying influence 

 of these storms had been on the top of the mountain 

 next day, his opinion would have been confirmed by 

 the greatly improved appearance of the atmosphere 

 after the storm. The thick veil which had hung in 

 the atmosphere for at least four days was gone, and 

 the distant mountains looked clear and distinct. Even 

 Hochgerrach, which is about 70 miles distant, was quite 

 distinctly seen during the whole day ; and the number 

 of particles fell to 400 per c.c. 



The question— Was the decrease in the dust, and the 

 improved appearance of the atmosphere on this occasion 

 due to the thunderstorm .-* — is then discussed. It is 

 shown that this conclusion is extremely doubtful. It is 



