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DUSSERAH, or DASARA

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Originally appearing in Volume V08, Page 716 of the 1911 Encyclopedia Britannica.
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DUSSERAH, or DASARA , a See also:Hindu new-See also:moon festival (some-times called Maha-navami), held in See also:October, and specially connected with ancestral See also:worship. In the native states, such as See also:Mysore, the rajas give public entertainments lasting for ten days, and especially invite See also:European officials to the festivities, which include See also:horse-racing, .athletic contests, and banquets. See J. A. See also:Dubois, Hindu See also:Manners, Customs and Ceremonies, p. 577. DUST, See also:earth or other See also:matter reduced to See also:fine dry and powdery particles; the word is See also:Teutonic and appears in such various forms as the Dutch duist, Danish dyst, for the dust of See also:flour or See also:meal, and in the older forms donst; the See also:modern See also:German Dunst, vapour, probably preserves the See also:original See also:form and meaning, that of something which can be blown about by the See also:wind. Atmospheric Dust.—The presence of dust in the See also:atmosphere has probably been known from the earliest ages, as prehistoric See also:man must have had plenty of opportunities of noticing it See also:lighting up the paths of sunbeams that penetrated his dark caves, yet it is only of See also:recent years that it has become the subject of scientific observation. Formerly it was considered as simply matter in the wrong See also:place, the presence of which had to be tolerated, but was supposed to serve no useful purpose in nature. It was not till the See also:year i88o that atmospheric dust came under scientific investigation, when it soon became evident that it played a most important See also:part in nature, and that instead of being a See also:nuisance to be got rid of, it added much to the comforts and pleasures of See also:life. The atmosphere is composed of a number of gases which have a nearly See also:constant proportion to each other, and of varying proportions of See also:water vapour. This vapour, constantly rising from See also:land and See also:sea, mixes with the gases in the atmosphere and so See also: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 See also:condition and form See also:cloud particles.

Before 188o it had always been assumed that when this condensation took place, the vapour molecules simply combined with each other to form the little globules of water, but J. Aitken showed that vapour molecules in the atmosphere do not combine with each other, that before condensation can take place there must be some solid or liquid See also:

nucleus on which the vapour molecules can combine, and that the dust in the atmosphere forms the nuclei on which the water-vapour molecules condense. Every cloud particle being grown See also: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 See also:air is cooled to the saturation point, and if there were no dust See also:present the condensation would not take place till the air was cooled far below that point, and become highly super-saturated; and when it did take place the condensation would be violent and result in heavy See also:rain-drops without the formation of what we know as cloud. This might be in some ways an See also:advantage, but living in such supersaturated air would have many disadvantages. The supersaturated air having no dust to condense on would condense on our clothes, the inside and outside walls of our dwellings, and on every solid and liquid See also:surface with which it came in contact. Many of the dust particles in the atmosphere which form the nuclei of condensation are extremely See also:minute, so small as to be beyond the See also:powers of the See also:microscope, and at first sight it might appear to be impossible to get any reliable See also:information as to their See also:numbers. But Aitken, having shown that water vapour must have a nucleus to condense on, saw that this placed in our hands the means of counting the dust particles in our atmosphere, and in 1888 showed how it could be done. As water vapour in the air condenses on the dust particles present and forms cloud particles, he showed that all that would be necessary would be to cause the dust particles to become centres of condensation, when they would be so increased in See also:size as to come within the range of an See also:ordinary magnifying See also:lens, and that by counting the cloud particles it would be possible to determine the number of dust particles. To carry out this See also:idea the air under examination was placed in an air-tight See also:receiver and saturated with water vapour. It was then See also:expanded by an air-See also:pump, and in this way cooled and condensation produced. The cloud particles so formed were allowed to fall on a See also:micrometer and their number counted by the aid of an ordinary See also:short-focussed lens.

Certain precautions are necessary in carrying out this See also:

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. When the number in the air tested exceeds that figure, the dusty air must he 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 See also:instrument at many places over the See also:world, and in no part of it has dustless air been found; indeed it is very rare to find air with less than too particles per c.c., whilst in most See also:country places the numbers rise to thousands, and in cities such as See also:London and See also:Paris the number may be as high as roo,000 to 150,000 per c.c. The See also:sources of dust particles in the atmosphere are numerous. In nature volcanoes See also:supply a large quantity, and the meteoric matter constantly falling towards the earth and becoming dissipated by the intense See also:heat produced by the See also:friction of the atmosphere keep up a constant supply. Large quantities of dust are also raised from the surface of the earth by strong winds, from dusty roads and dry See also:soil, and there is See also:good See also:reason for supposing that large quantities of See also:sand are carried from the deserts by the wind and transported See also:great distances, the sand, for instance, from the See also:desert of See also:Africa being carried to See also:Europe. It is, however, to artificial causes that most of the dust is due. The burning of See also:coal is the See also:principal source of these, not only when the coal is burned with the See also:production of See also:smoke, but also when smokeless, and even when the coal is first converted into See also:gas and burned in the most perfect forms of See also: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. 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 See also: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 See also:wash the others out of the atmosphere. We may therefore look on all uninhabited areas of the earth as purifying areas, and their purifying See also:power seems to depend partly on their extent, but principally on their rainfall.

The following table illustrates the purifying effect of some of these areas obtained from the results of hundreds of observations. The areas referred to are: (t) Mediterranean Sea, the observations being made on the See also:

south See also:coast of See also:France on the air blowing inshore; (2) the See also:Alps, the observations being made on the Rigi See also:Kulm; (3) the See also:Highlands of See also:Scotland, the observations being made at various places; and (4) the See also:Atlantic Ocean, the observations being made on the See also:west coast of Scotland, when the wind blew from the ocean. Mediterranean. Alps. Highlands. Atlantic. Mean of lowest 891 381 rot 72 Mean of number 1611 892 552 338 These numbers are all See also:low for atmospheric dust, much See also:lower than in air from inhabited areas. On the Rigi Kulm, for instance, the number was sometimes over ro,000 per c.c. when the wind was from inhabited areas and the See also:sun causing ascending currents; and at the same place as the Atlantic air was tested the numbers went up to over 5000 per c.c. when the wind blew from the inhabited areas of Scotland, though the distance to the nearest was over 6o m. E. D. Fridlander t made many observations on the dust of the atmosphere with the same instrument as employed by Aitken. In See also:crossing the Atlantic he got no low numbers, always over 2000 per c.c., but in the Gulf of St See also:Lawrence he got a See also:reading as low as 28o per c.c.

In crossing the Pacific the lowest obtained was 245, in the See also:

Indian Ocean 243, in the Arabian Sea 280, in the Red Sea 383, and in the Mediterranean 875 per c.c. He has also made observations in See also:Switzerland. The lowest number obtained by him was in the air at the See also:top of the Bieshorn, i3,600 ft. above sea-level, where the number was as low as 1S7 per c.c. See also:Professor G. Melander2 of See also:Helsingfors studied the dust in the atmosphere. His observations were made in Switzerland, See also:Biskra in the See also:Sahara, See also:Finland, the See also:borders of See also:Russia, and in See also:Norway; but in none of these places were low numbers observed. The minimum numbers were over 300 per c.c., while maximum numbers in some cases went high. Aitken when observing on the Rigi Kulm noticed during some 1 " Atmospheric Dust Observations from various parts of the World," Quart. Journ. See also:Roy. Met. See also:Soc.

(See also:

July 1896). 2 La Condensation de la vapeur d'eau clans l'atmosphere (Helsingfors, 1897) conditions of See also:weather that there was a daily variation in the number of particles, a maximum near the hottest part of the See also:day and a minimum in the See also:morning, and attributed the rise in the numbers to the impure air of the valleys rising on the sun-heated slopes of the See also:mountain or driven up by the wind. A. Rankin, at the See also:Ben See also:Nevis See also:observatory, also observed this daily variation, and his observations also indicate a yearly variation at that station, the numbers being highest in See also:March, See also:April and May. This may possibly be due to small rainfall in these months, but more probably to the fact that south-easterly winds See also:blow more frequently during these months on Ben Nevis than at any other See also:season, and these winds bring the impure air from the more densely inhabited parts of the country. Without atmospheric dust not only would we not have the glorious cloud scenery we at present enjoy, but we should have no haze in the atmosphere, none of the atmospheric effects that delight the artist. The See also:white haze, the See also:blue haze, the See also:tender sunset glows of red, See also:orange and yellow, would all be absent, and the moment the sun dipped below the See also:horizon the earth would be in darkness; no See also:twilight, no after-glows, such as those given some years ago by the volcanic dust from See also:Krakatoa; none of the See also:poetry of eventide. Why, it may be asked, is this so ? Simply because all these are due to matter suspended in the air, to dust. 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. Turning now to the investigations on this point, Aitken has shown that there is no See also:evidence to indicate that water vapour has any hazing effect, and shows that the haze is entirely due to dust, the See also:density of the haze increasing with the increase in the number of dust particles in the air, and also with the relative humidity; but the humidity does not See also:act as vapour, but by condensing on the dust and increasing the size of the particles, as it is not the amount of vapour present but the degree of saturation that affects the result; the more saturated the air, the more vapour is condensed on the particles, they so become larger and their hazing effect increased. The relation of haze or transparency of the air to the number of dust particles was observed on five visits to the Rigi Kulm.

The visibility of Hochgerrach, a mountain 70 M. distant from the Rigi, was used for estimating the amount of haze when the air was clear. During the visits this mountain was visible thirteen times, and it was never seen except when the number of particles was low. On eight occasions the mountain was only one-See also:

half to one-fifth hazed, and on these daysthe number of particles was as low as from 326 to 850 per c.c. It was seen five times when the number was from 950 to 2000 per c.c., but the mountain on these occasions was only just visible, and it was never seen when the number was a little over 2000 per c.c. It has been pointed out that the relative humidity has an effect on the dust by increasing the size of the particles and so increasing the haze. It was therefore necessary in working out the dust and haze observations made at the different places to arrange all the observations in tables according to the wet-bulb depressions at the See also:time. All the observations taken when the wet-bulb depression was between 2° and 4° were put in one table, all those when it was between 40 and 70 in another, and all those when it was over 7° in a third. It should be here noted that when the dust particles were counted and the wet and dry bulb observations taken, an estimate of the amount of haze was also made. This was done by estimating the amount of haze on a mountain at a known distance. Suppose the mountain to be 25 M. distant, and at the time to be one-half hazed, then the limit of visibility of the mountain under the conditions would be 50 m., and that was taken as the number representing the transparency of the atmosphere at the time. In the tables above referred to along with the number of particles was entered the limit of visibility at the time; when this was done it was at onceseen that as the number of particles increased the limit of visi. bility decreased, as will be seen from the following short table of the Rigi Kulm observations when the wet-bulb depression was between 2° and 40 When the number of particles is multiplied by the limit of visibility in the tables a fairly constant number C. is obtained; see preceding table. All the observations taken at the different places were treated in a similar manner and the means of all the observations at the different humidities were obtained, and the following table gives the mean values of C. at the different wet-bulb depressions of all the observations made at the different places.

Wet-bulb depression . 2° t0 40 4° to 7° 7° and over Mean values of C. . 76,058 105,545 141,148 From the above table it will be seen that as the dryness of the air increased it required a larger number of particles to produce a See also:

complete haze, nearly See also:double the number being required when the wet-bulb depression was over 7° than when it was only from 2° to 4°. To find the number of particles required to produce a complete haze, that is, to render a mountain just invisible, all that is necessary is to multiply the above constant C. by 160,930, the number of centimetres in a mile, when this is done with the observations made in the West Highlands we get the numbers given in the following table: Wet-bulb depression. Number of Particles to produce a complete haze. 2° to 4° 12,500,000,000 4° to 7° 17,100,000,000 7° to 10° 22,600,000,000 The above table gives the number of particles of atmospheric dust in a See also:column of air having a See also:section of one centimetre square, at the different humidities, required to produce a complete haze, that is, to make a distant See also:object invisible, and is of course quite See also:independent of the length of the column. In making these dust and transparency observations three things were noted: 1st, the number of particles; 2nd, the humidity; and 3rd, the limit of visibility. From the results above given, it is evident that if we now know any two of these we can calculate the third. Suppose we know the limit of visibility and the humidity, then the number of particles can be calculated by the aid of the above tables. To show the hazing effects of dust it is not, however, necessary to use a dust See also:counter. Aitken for some years made observations on the haze in the air at See also: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 See also:peculiar See also:distribution of the See also:population surrounding it.

The whole See also:

area fiom west, See also:north-west to north, is very thinly populated, while in all other directions it is densely populated. 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. 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 to times more hazed than if there were no inhabitants in the country. (J. A.*) Date. Lowest Highest Mean Limit of C. Visibility in Number. Number. Number. See also:Miles. 19th May 1891 428 690 559 150 83,85o Mean 22nd May 1889 • 434 85o 642 See also:loo 64,200 } 16th May 1893 1225 2600 1912 40 77,480 ) 75 176.

End of Article: DUSSERAH, or DASARA

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