John Aitken

“ It is evident that if there were not some purifying process in nature there would be a tendency for the dust particles to increase in numbers, because though some dust particles may fall out of the air, many of them are so small they have but little tendency to settle, but by becoming centres of cloud particles they are carried downwards to the earth, and, further, these when showering down as rain tend to wash the others out of the atmosphere. ”

“ If, however, the facts and arguments should fail to carry conviction, I trust that the effort I have made to solve this somewhat difficult and perplexing question may not prove altogether fruitless; for should no other result follow than that of attracting the attention of other observers to the subject, so that eventually a satisfactory solution may be arrived at, my object will, at all events to some extent, have been attained. ”

“ The watersheds of the Walsden and Cliviger valleys are at a comparatively low level, that of the first having an altitude of 627 feet above sea-level, and being eaten into to a depth of 330 feet below the shoulder, or first platform bounding the valley [4] ; whilst that of the second is crossed at an elevation of 768 feet, and is excavated to a depth below the immediate country of 475 feet, a depression which would be greatly increased if the summit-level of the surrounding hills were taken as the datum-line for measurement. ”

“ The burning of coal is the principal source of these, not only when the coal is burned with the production of smoke, but also when smokeless, and even when the coal is first converted into gas and burned in the most perfect forms of combustion. It results from this that while in the air over the uninhabited parts of the earth and over the ocean the number of particles is small, being principally produced by natural causes or carried from distant lands, they are much more numerous in inhabited areas, especially in those where much coal is burned. ”

“ Large quantities of dust are also raised from the surface of the earth by strong winds, from dusty roads and dry soil, and there is good reason for supposing that large quantities of sand are carried from the deserts by the wind and transported great distances, the sand, for instance, from the desert of Africa being carried to Europe. It is, however, to artificial causes that most of the dust is due. ”

“ On the approach of the long winter which preceded the period when ice overspread the county of Lancaster and the whole of the northern part of our island, the severity of the climate would gradually increase; and during this time large accumulations of snow would take place, and ice be formed in deep and sheltered situations like those presented by the Walsden, Cliviger, and Todmorden valleys. ”

“ The atmosphere is composed of a number of gases which have a nearly constant proportion to each other, and of varying proportions of water vapour. This vapour, constantly rising from land and sea, mixes with the gases in the atmosphere and so long as it remains vapour is invisible, but when it becomes cooled by the actual processes in nature the vapour tends to condense to the liquid condition and form cloud particles. ”

“ A great number of facts, and the statements of various authorities as to the altitude of high-level drift on the western slopes of the Pennine chain, indicate a general elevation of from 1100 to 1200 feet, whilst scattered boulders and pebbles of travelled rocks may be detected at various places in the locality some 200 feet in excess of that; and I contend that these more elevated stray boulders and pebbles owed their origin to icebergs during the interglacial period, and not to the period of land ice. ”

“ To show the hazing effects of dust it is not, however, necessary to use a dust counter. Aitken for some years made observations on the haze in the air at Falkirk by simply noting the direction of the wind, the wet-bulb depression at the time, and the transparency of the air. Falkirk is favourably situated for such observations owing to the peculiar distribution of the population surrounding it. The whole area from west, north-west to north, is very thinly populated, while in all other directions it is densely populated. ”

“ Every cloud particle being grown round a dust nucleus thus has a dust particle in it. The presence of dust in the atmosphere allows the condensation of the vapour to take place whenever the air is cooled to the saturation point, and if there were no dust present the condensation would not take place till the air was cooled far below that point, and become highly supersaturated ”

“ Atmospheric Dust.—The presence of dust in the atmosphere has probably been known from the earliest ages, as prehistoric man must have had plenty of opportunities of noticing it lighting up the paths of sunbeams that penetrated his dark caves, yet it is only of recent years that it has become the subject of scientific observation. ”

“ Starting from this centre, three valleys radiate in different directions, in lines almost equidistant from each other:—one stretching off through Walsden to Littleborough for a space of about 5 miles in a direction a little W. of S.; another taking a N.W. course through the sinuous gorge of Cliviger to Burnley, a distance of about 8 miles; whilst the third, the valley of the Calder, takes an E.N.E. direction. ”

“ Water has no such effects as long as it is a vapour, and if it condensed without the presence of dust, the particles would be far too few to give any appreciable effect and too heavy to remain in suspension. ”

“ It was found that the air from the thinly inhabited parts, that is, the north-west quadrant, was nine times clearer than the air from other directions with the same wet-bulb depression, and that the density of the haze was directly proportional to the density of the population of the area from which the wind blew. ”

“ To carry out this idea the air under examination was placed in an air-tight receiver and saturated with water vapour. It was then expanded by an air-pump, and in this way cooled and condensation produced. The cloud particles so formed were allowed to fall on a micrometer and their number counted by the aid of an ordinary short-focussed lens. Certain precautions are necessary in carrying out this process. There must not be more than 500 particles per cubic centimetre of air, or all the particles will not form nuclei, and will not therefore be thrown down as cloud particles. ”

“ In support of the theory that a large body of moving ice meeting with resistance sufficiently powerful can be either entirely arrested, or its various portions be endowed with varying and diverse motions, we have the authority of the Duke of Argyll, Prof. ”

“ These observations also showed that the transparency of the air increases with the dryness, being 3.7 times clearer when the wet-bulb depression is 8° than when it is only 2°, and that the air coming from the densely inhabited parts is about 10 times more hazed than if there were no inhabitants in the country. ”

“ When the number in the air tested exceeds that figure, the dusty air must be mixed with such a quantity of dustless air as will reduce the number below 500 per c.c., and the correct number in the air tested is obtained by allowing for the proportion of dustless air to dusty air, and for the expansion necessary for cooling. ”

“ Thousands of tests of the atmospheric dust have been made with this instrument at many places over the world, and in no part of it has dustless air been found; indeed it is very rare to find air with less than 100 particles per c.c., whilst in most country places the numbers rise to thousands, and in cities such as London and Paris the number may be as high as 100,000 to 150,000 per c.c. ”