34 



NATURE 



\^Nov. 8, r 



of general scientific knowledge, and of special knowledge 

 in, at least, some one branch, as well as the necessity 

 of a real passion for travelling, Prjevalsky adds : — 

 " Moreover, he must be an excellent shooter, and, still 

 better, a passionate hunter. He must not despise any 

 hard manual work, as, for instance, the saddling of horses 

 and camels, the packing of luggage, and so on — in short, 

 he must never be a * white-handed' person ; he must not 

 have habits of luxury ; and he must have a pleasant, 

 lenient character, which will soon acquire for him the 

 friendship of his travelling companions." In these sen- 

 tences he characterized himself. To renounce, if necessary, 

 every comfort ; to live the life of the other members of the 

 expedition, without any distinction between the scientific 

 staff and the simplest soldiers or Cossacks ; to sleep in the 

 same tent, to eat the same food, and to do the same work 

 as the rest— such were Prjevalsky's rules. We must add 

 also that, especially dunng his first two journeys, his 

 relations with the natives were of the most friendly 

 character. He carefully avoided any conflict with them ; 

 and when it happened once, during his first journey, that 

 the natives were hostile to him, and this hostility might 

 have ended in an armed conflict, he preferred to win their 

 respect by the following stratagem. He and his three 

 men— all four admirable sharp-shooters — opened a fire 

 from their breech-loading rifles upon the carcass of a 

 horse, from a great distance. In two minutes they had 

 discharged thirty bullets each, and they advised the 

 Mongols to see if any bullet had touched the carcass. 

 The Mongols rushed, of course, to the carcass, and, to 

 their great astonishment, after hard work with their 

 knives, discovered most of the 120 bullets in it. They 

 did not fail, after this, to treat their visitors properly. 



It is impossible to mention Prjevalsky's name without 

 being reminded of his travelling companions. He him- 

 self so often expressed his gratitude to them, and he always 

 wrote with so much sympathy about their common ex- 

 periences, that we shall only be carrying out his wish in 

 stating that Lieut. Pyevtsoff during Prjevalsky's first two 

 journeys, Lieut. Roborovsky in the last two journeys, and 

 M. Kozloff during the fourth, have their lull share in 

 what Prjevalsky modestly described as his " scientific 

 reconnoitrings" in Central Asia. Their portraits, as well 

 as his own, are given in his last viork. P. K. 



SMOKE IN RELA TION TO EOGS IN LONDON} 



T ON DON fogs are produced by the mechanical 

 -*— ' combination of particles of water with particles of 

 coal or soot, and require for their fullest development the 

 following conditions : a still air, a temperature lowest at 

 or near the ground in comparison with an elevation of 

 some hundreds of feet, saturation or partial saturation of 

 the air within a moderate distance of the ground, absence 

 of clouds overhead, and free radiation into space. The 

 artificial darkness and peculiar colouring occur with 

 greatest effect at times when a very large quantity of coal 

 is being burnt in domestic fireplaces, and cannot as a rule 

 prevail during the night between 10 p.m. and 5 a.m., or to 

 any great degree in warrn summer weather. The early 

 hours of summer mornings are the only ones in which 

 clear views of the whole city are possible.' Next in clear- 

 ness come fine Sunday afternoons in summer, when fires 

 are allowed to go down. The hours of greatest density 

 are those following the time of greatest cold on the 

 earth's surface and of the lighting of large numbers of 

 kitchen and other fires. Thus about 8 to 10 a.m. is 

 frequently the period of thickest and darkest fogs. It 

 may be noted that on Sundays, when factory fires are 

 inoperative, fogs in winter have been densest : on one 

 Christmas Day there was absolute darkness during the 



' Abstract of an Address delivered by the Hon. F. A. R. Russell, on 

 March I, 1888, under the auspices of the Smoke Abatement Institution. 



whole day, thus showing the dependence of light-absorb- 

 ing matter on kitchen and domestic fires. Many distinct 

 conditions may alter the time of maximum density. 



The formation of a London fog appears to take place 

 as follows. An ordinary thick white fog c.overs the city, 

 say at 6 a.m. ; about a million fires are lighted soon after 

 this hour, and the atmosphere becomes charged with 

 enormous volumes of smoke— that is, the gases of com- 

 bustion bearing carbonaceous particles. Now, these 

 particles, as soon as they are cooled to the temperature 

 of the air, or below it, begin to attach to themselves the 

 water spherules already present and visible, and vapour 

 may also be condensed on the particles. A thick layer of 

 these united particles prevents light from penetrating it, 

 and a very small quantity of finely divided carbon may 

 stop the bright sunshine altogether, like the film of soot 

 on a smoked glass. The invariable redness of the 

 sun through smoke seems to show that the majority of 

 particles are comparable in diameter to the length of a 

 wave of blue light. Smoke prevents the warmth of the 

 oblique sunshine from reaching and evaporating the white 

 fog near the ground, and the white fog continues to radiate 

 towards space and towards the ground, if colder than 

 itself, without receiving compensation from the solar rays. 

 A difference of lo'' has occasionally been noted between 

 thermometers at 4 feet and 100 feet above the ground, the 

 upper one always being the warmest in fogs. Carbon is 

 a good radiator, and tends from this cause to keep itself 

 cold by radiation into space, and thus to accumulate 

 vapour from the air, like the dewy surface of the earth. 

 The importance of a clear sky and a dry upper air in 

 promoting fogs in this respect is obvious. 



It has been supposed, quite erroneously as I believe- 

 that, as even without any visible smoke an enormous 

 quantity of fine invisible dust exists in the London 

 atmosphere, the abolition of coal smoke would fail to 

 give us freedom from dark fogs. Proof is altogether 

 wanting that ordinary invisible dust in cities or plains is 

 of a kind to create an abnormal amount of fog, or to- 

 produce those dismal obfuscations of which tarry carbon 

 is capable. Paris, as long as it burnt wood and charcoal,. 

 was free from idiosyncratic fogs ; so are the wood and 

 anthracite-burning cities of the United States ; and so are 

 the towns of South Wales, where anthracite is the com- 

 mon fuel. If London Mere to cease using fuel in the 

 solid form, it would be as free from fog as the surrounding 

 country. 



The accumulation which produces the worst and most 

 dangerous fogs in London cannot, as a rule, proceed for 

 many hours without disturbances arising which tend to 

 reduce their importance. In winter the warmth of the 

 air exceeds by 2" or 3° that of the surrounding country,, 

 consequently an upward current is started, which rises 

 to some altitude, and then flows away, bearing with it a 

 stream of murky cloud ; a circulation of air consequently 

 takes place in the lower strata. If, however, the cold of 

 the lower layers greatly surpasses that of the strata some 

 hundreds of feet higher, and if the sun dissipates the fog 

 in the surrounding country, thus making the environs 

 warmer than London itself, the black fog may very likely 

 remain on the town all day. For this reason, on a fine 

 cold still morning, with a bright sun, and temperature 

 near the dew-point, persons arriving from the country are 

 pretty sure to find a black fog in town between 10 and 

 12 a.m. The finest winter days are nearly always very 

 bad ones in London, unless there be any wind between 

 the surface and an altitude of 1000 feet, or the dryness be 

 unusual for the time of year. 



When the air is very dry near the surface, no dense fog 

 is formed in London, and when very wet, streaming with 

 fog, in the country, little fog occurs in London. The dry 

 warm surfaces of the houses themselves, and the elevation 

 of temperature above the dew-point, prevents wet fogs 

 from reaching anything like the density they attain in the 



