September, 1902.] 



KNOWLEDGE 



197 



not infri'ijueutly in definite clouds which yield to influence 

 of breeze, which are higher in dry and lower in moist 

 weather, and which are largely washed out of the air by 

 summer showers. I have oftou found that at some height, 



Pliotoa^raph of New Oxford Street, London, showing St. Giles' 



Church. Trails of light from passing vehicles alone visible. 



Six hours' exposure. 



generally' about (iOt'O feet, it is possible to surmount the 

 haze and look down upon its surface as though the grosser 

 matter in suspension had a definite upper limit. Above 

 this limit the day sky wears a darker blue and at night 

 the stars redouble their splendour. 



As to the actual nature of the dust collected and 

 examined I have certain information to give. Dr. Angus 

 Smith, who used methods very similar to my own, 

 discovered besides formless matter actual crystals in the 

 air under certain circumstances, metallic matter in the 

 form of dust resembling rolled or torn plates of iron in 

 railway carriages, and so on. High over London I have 

 found nothing of this nature, but I have met with a great 

 quantity of such dust as will settle by night on the 

 furniture of a room together with much of altogether 

 larger dimensions resembling chaff, filaments, and woollen 

 fibre ; such dust, in fact, as \tould arise off the thorough- 

 fares in dry weather, or from the sweeping out of houses. 

 But such dust was more in evidence at from 3Ut;<) to .50(tO 

 feet above the house-tops than nearer the ground. It 

 would seem to accumulate at the higher levels, and then 

 in anti-cyclonic conditions wheu mixed with smoke and 

 under circumstances favouring fog one would like to know 

 exactlv what part this dust-laden canopy plays. 



When the fog comes down it often begins with the down- 

 rush of a dense mass which when viewed from a lofty 

 building seems to swamp the lower roofs as with a murky 

 wave. This presently lifts again into space, but only to 

 return again and again till it finally settles down and the 

 fog becomes uniformly dense and general. These conditions 

 I have photographed from various heights u|> to that of 

 the golden gallery of St. Paul's, but the results, though 

 highly instructive, do not lend themselves to reproduction. 



As to the far travel of London smoke when it is not 

 detained in fogs over the city, there is jilenty of evidence 

 forthcoming. In days of bright sunshine in the open 

 countrv the fairest and most-favoured suburbs of the West 

 End may be partially over-clouded if au east wind causes 

 London smoke to set in that direction. This same smoke 



may be actually seen at thirty miles to leewai'd, and when 

 it drifts away as London fog it mav be smelt for twice 

 that distiince. The smoke from the Black Country has 

 been stated to be carried down the wind over vast are.is, 

 a fact which must of course be open more or less to con- 

 jecture, but I can speak from ocular eviden<'e of the 

 extraordinary passage of smoke travelling far and wide, 

 but not ascending more than a very moderate height 

 into the sky. It was on a calm clear night last summer 

 that I made a night voyage over London, starting at 

 8 a.m. With gradually increasing altitude, Fulham was 

 crossed, then the suspension bridge at Battersea, ne.\t 

 Lambeth and Peckham, and so out over Kent. As far 

 as could be discerned, London proper at all heights was 

 as free from smoke as can be conceived, but wlieu the 

 town was left far in our wake our course took us over 

 some cement works, whose chimneys were pouring 

 volumes of smoke into the sky. Tiiis smoke, however, 

 as viewed from our level of 4500 feet, seemed to rise no 

 considerable height anywhere into the atniosjihere; on 

 the i;ontrai-y, it rapidly spread into a low-lying layer, and 

 then travelled outwards without sensible thinning away 

 as far as the eye could reach. At twenty miles distant 

 it was on the earth, still making itself uui>leasantly 

 manifest. 



In the above c<ase, of course, the smoke was drifting 

 away as fast as formed, and no dense accumulation over 

 the land was possible. Had the air been quiescent the 

 case would have been very different, and it may not be 

 diflicult to form a reasonable conception of what the air 

 over London might be found to be if explored by balloon 

 at a time of visitation of fog. As to what may be observed 

 from the highest accessible point of St. Paul's I can record 

 some experiences. On many days of moderate fog you 

 may be able to climb above the actual fog limits, but in 

 the exceptional fogs preceding last Christmas this was not 

 so. Under these circumstances what are the actual 

 dimensions and conditions of the fog ? In one of the early 

 numbers of Knowledcje, R. A. Proctor points out by 

 mathematical reasoning, depending on the amount of light 

 that even in densest fogs actually does struggle through 

 from the clear sky above, that the depth of the fog layer 

 cannot be very great. And this seems probable on every 

 account. Particles of carbon, being good radiators, must 

 readily reach the dew point, and becoming heavy with 

 deposition of moisture, must in circumstances of fog be 

 incapable of rising high into the atmosphere. 



But if the fog be shallow, it is moreover presumably 

 in a condition" of instability. The fog itself checking 

 radiation, there must be considerable difference of tem- 

 perature between its lower and upper limits. Nor is this 

 all ; the alternations of warm and cold air streams so 

 often recorded in balloon ascents, and also deduced from 

 kite experiments, are in my experiences, which are now 

 not few, very frequently met with above a cloud layer. 

 Such alternations seeming indeed to be often independent 

 j of visible cloud, and in ascents over London they are 

 I i)ossibly more noticeable than over the open country. 

 (This might indeed be inferred as probable when the 

 i variations of radiation from a town like London are con- 

 sidered.) Under these circumstances we may suppose 

 that some small source of disturbance might be sufficient 

 to cause the densest fog to break away upwards, or, in the 

 common phrase, to " lift " in that magical manner so often 

 observed. 



One other factor operative in the dispersal of fogs 



I should not be left out of account, namely, the diverse 



I upper currents so often encountered over Loudon. Charles 



I Green called attention to these as far back as 1838, when 



at less than half a mile above the earth his course 



