February 25, 19 15] 



NATURE 



699 



and other points will be discussed in our memoir on 

 the London femora now at press. I write at present 

 at this length merely to indicate that in suggesting to 

 Dr. Lee to illustrate her tables on Dr. Parsons 's mate- 

 rial, I had in view many observational points which 

 Dr. Parsons, like not a few of our English critics, 

 considers we have made no attempt to appreciate — 

 being "mere mathematicians." Karl Pearson. 



Galton Laboratory, University of London. 



A Remarkable Dry Fog in the East Indian Archipelago. 



The observations made on Ben Nevis upon the 

 transparency of the air have shown very clearly that 

 with increasing relative humidity the transparency is 

 •diminishing, a consequence of the increasing volume 

 of the light-scattering particles, from which it may be 

 concluded that the particles are of hygroscopic char- 

 acter. 



The influence of humidity on the size of these 

 particles was obvious in a very striking way in the 

 dense dry fog that towards the end of the dr}' mon- 

 soon of 1914 spread out over the East Indian archi- 

 pelago, causing much trouble to navigation. 



A short description of this phenomenon is given 

 below. The fog had a grey colour, and was most 

 intense in the month of September, and especially in 

 October, and disappeared when the rains fell. In 

 its most typical form it appeared in Sumatra, in the 

 neighbourhood of Ambon and the islands to the south- 

 east. 



In Sumatra it was observed to the north as far as 

 Medan (Deli), the Karo plateau near the Toba lake 

 and Tapanoeli, to the south as far as Benkoelen and 

 the highlands of Kerintji. It was not observed in the 

 open ocean outside the Mentawei Islands. 



In the morning the sun was not visible before 

 eight o'clock, when it appeared as a dim red disc. 

 Objects in general were barely visible at 500 metres' 

 distance, and on the thickest days invisible within 

 half this distance ; the mountains had disappeared 

 entirely from view. 



In Ambon the sea was sometimes invisible from the 

 lighthouse, which is situated close to the shore, and 

 140 metres above the sea. Also in the straits between 

 Sumatra and Borneo, and the west and east coast of 

 Borneo, the fog was very thick, and though in some- 

 what less degree it was also observed in the northern 

 part of the Strait of Makasser, on the north coast of 

 New Guinea (in these regions, however, it was more 

 or less mixed up with smoke from forest fires, which 

 caused local intensification), on Sumatra's west coast 

 north of Padang and Malakka Strait. 



Though in the other parts of the archipelago the 

 transparency of the air was greater, it was not quite 

 clear, on account of the blue haze that is common to 

 the dry season. 



A similar phenomenon (described by Dr. van 

 Bemmelen, Meteorl. Zeiischr., August, 1905), perhaps 

 still more intense, was observed in the very dry year 

 1902 ; it then extended to the southern part of the 

 China Sea and to the south-east so far as the 

 Tanimber Islands, the region of greatest intensity 

 being about the same as in 1914. It was also reported 

 from the abnormally dry vears 1885, 1888, 1891, and 

 1896. In Karimata Strait it is a common pheno- 

 menon. 



Now it is very remarkable that the southern 

 tx)undary of the fog coincides rather closely with a 

 well-defined line, that separates the legion of prac- 

 tically no rainfall from that of greater humidity where 

 light' rains begin to appear. In 1914 this line ran in 

 the months of September and October over the south- 

 east point of Sumatra, along Billiton and Borneo's 



south coast, through Makassar Strait, along North 

 Celebes, and south-eastwards along the south coast of 

 New Guinea, Ambon, and the region south-east of it 

 lying as an oasis of light rainfall in the dry region. 



From this it is evident that it is the hygroscopic 

 particles that form the blue haze in the south-eastern 

 part of the archipelago, which, entering into moister 

 regions and increasing their radius, give rise to the 

 appearance of the grey fog. 



Physical laws fully account for the fact that the fog 

 is a mere intensification of the haze, though the 

 rather fast transition of the light blue haze to the 

 much thicker grey fog gives the impression of a new 

 phenomenon springing into existence. 



So long as the particles are much smaller than the 

 wave-length, the intensity of the scattered light in 

 relation to that of the light falling upon it, may be 

 represented by I=ar'/R*A*, when r is the radius of 

 the particle, R the distance from the observer, and A 

 the wave-length. When r is increasing, the ordinary 

 reflection ultimately appears, and I = /3r*/R*A'. 



From the method of dimensions may be concluded 

 (conf. Rayleigh, "Theory of Sound," II., p. 152), I 

 being a simple ratio and a and /3 passing gradually 

 and rather slowly into each other as r increases, that 

 as the exponent (n) of A is changing from 4 to o the 

 exponent (m) of r changes from 6 to 2. Therefore, so 

 long as n is large enough to produce a colourmg of 

 the haze, m will cause a very rapid increase of the 

 intensity of the scattered light, which may account 

 for the rather sudden change of the blue haze to the 

 grey fog. 



It is not in the first place the greater number of 

 particles that account for the difference between the 

 transparency of the air in the dry and wet season, but 

 their hygroscopic character. On December 23, 1914, 

 when the wet season had already set in, I counted 

 with Aitken's apparatus at Batavia 50,000 particles in 

 I cub. cm., the relative humidity being 84 per cent., 

 and no trace of haze was to be seen. However, on 

 the Ardjoeno (at 2500 metres' height) on October 29, 

 towards the end of the diy season, their number was 

 only 1800 in i cub. cm., and not withstanding the 

 humidity was only 40 per cent., the surroundings were 

 enveloped in an intense blue haze. Especially these 

 very hygroscopic particles, when entering moister 

 regions, will be susceptible to a strong increase of 

 volume, and therefore will be the principal producers 

 of the fog. 



When we put the question where those highly 

 hygroscopic particles come from that are responsible 

 for the phenomenon described above, it may be re- 

 marked that for the greater part they do not form 

 during the short journey of the trade wind over the 

 archipelago. This takes only a few days, whereas 

 the fog appears after many months of excessive 

 drought. Two causes may be mentioned to which 

 they owe their existence, viz., burning processes in 

 Australia, which are also most frequent after long- 

 prolonged droughts; and, secondly, which I take for 

 the greater influence, the formation from the com- 

 ponents of the air, ozone, nitrogen, etc., during the 

 slow descending movement in the .Australian anti- 

 cyclone, in which the air probably takes several 

 months to reach the layers near the surface of the 

 earth. ' C Braak. 



Batavia, January, 1915. 



A Penalty on Research. 



If Sir Wm. Ramsay was refused a rebate of the 

 duty on alcohol I could scarcely expect better treat- 

 ment. Nevertheless, I should like to add to his pro- 

 test, and complaint After a correspondence lasting 



