A city (named from the Latin, civitas), in a general sense is a town holding, from extent of population, favourable situation, or other causes, a leading place in the community in which it is situated. Popularly, the name is also used, both in Britain and France, to designate the old and central nucleus as distinguished from the suburban growths of large towns. The ecclesiastical sense of the term city is a town which is, or has been, the see of a bishop. This seems to be the historical use of the term in England, and still possesses some authority there, but to a considerable extent it has been superseded by the wider one. In America the application of the term is dependent upon the nature and extent of the municipal privileges possessed by corporations, and a town is raised to the dignity of a city by special charter. Generally the term implies the existence of a mayor at the head of the municipality.
A comet is a small body orbiting the sun on an elliptical path with a long tail of dust and ice.
Some comets are only visible by the aid of the telescope, while others can be seen by the naked eye. In the latter case they usually appear like stars accompanied with a train of light, sometimes short and sometimes extending over half the sky, mostly single and more or less curved, but sometimes forked. In a comet which appeared in 1744 the train was divided into several branches, spreading out from the head like a fan. The train is not stationary relatively to the head, but is subject to remarkable movements. The direction in which it points is always opposite to the sun, and as the , comet passes its perihelion the train changes its apparent position with extraordinary velocity. The head of the comet is itself of different degrees of luminosity, there being usually a central core, called the nucleus, of greater brilliancy than the surrounding envelope, called the coma.
Comets were long regarded as supernatural objects, and usually as portents of impending calamity. Tycho Brahe was the first who expressed a rational opinion on the subject, coming to the conclusion that the comet of 1577 was a heavenly body at a greater distance from the earth than that of the moon. The general law of the motion of bodies, as well as his own observations on the comet of 1680, led Isaac Newton to conclude that the orbits of the comets must, like those of the planets, be ellipses, having the sun in one focus, but far more eccentric; and having their aphelions, or greater distances from the sun, far remote in the regions of space.
This idea was taken up by Halley, who collated the observations which had been made of all the twenty-four comets of which notice had been taken previous to 1680. The results were very interesting. With but few exceptions the comets had passed within less than the earth's shortest distance from the sun, some of them within less than one-third of it, and the average about one-half. Out of the number, too, nearly two-thirds had had their motions retrograde, or moved in the opposite direction to the planets. While Halley was engaged on these comparisons and deductions the comet of 1682 made its appearance, and he found that there was a wonderful resemblance between it and three other comets that he found recorded - the comets of 1456, of 1531, and of 1607. The times of the appearance of these comets had been at very nearly regular intervals, the average period being between seventy-five and seventy-six years. Their distances from the sun, when in perihelion, or when nearest to that luminary, had been nearly the same, being nearly six-tenths of that of the earth, and not varying more than one-sixtieth from each other.
The inclination of their orbits to that of the earth had also been nearly the same, between 17 degrees and 18 degrees; and their motions had all been retrograde. Putting these facts together, Halley concluded that the comets of 1456, 1531,1607, and 1682 were reappearances of one and the same comet, which revolved in an elliptic orbit round the sun, performing its circuit in a period varying from a little more than seventy-six years to a little less than seventy-five; or having, as far as the observations had been carried, a variation of about fifteen months in the absolute duration of its year, measured according to that of the earth. For this variation in the time of its revolution Halley accounted upon the supposition that the form of its orbit had been altered by the attraction of the remote planets Jupiter and Saturn as it passed near to them; and thence he concluded that the period of its next appearance would be lengthened, but that it would certainly reappear in 1758 or early in 1759. As the time of its expected reappearance approached, Clairaut calculated that it would be retarded 100 days by the attraction of Saturn, and 518 by that of Jupiter, so that it would not come to the perihelion, or point of its orbit nearest the 500 sun, until the 13th of April, 1759.
It actually reached its perihelion on the 13th of March, 1759, being thirty days earlier than he had calculated. Along with the period of this comet and its perihelion distance, the magnitude and form of its path were also calculated. Estimating the mean distance of the earth from the sun at 95,000,000 miles (the number which was at that time considered as the true one), the mean distance of the comet was calculated to be 1,705,250,000 miles; its greatest distance from the sun, 3,355,400,000; its least distance, 55,100,000; and the transverse or largest diameter of its orbit, 3,410,500,000. This comet, therefore, is a body belonging to the solar system, and quite beyond the attraction of any body which does not belong to that system; and as this is determined of one comet, analogy points it out as being the case with them all. In 1835 it again returned, being first seen at Rome, on August the 5th, and from that time continued to be observed until the end of the year in Europe, and through a great part of spring 1836 in the southern hemisphere.
The comet denominated Encke's comet, which has made repeated appearances, was first observed in 1818, and was identified with a comet observed in 1786, also with a comet discovered in 1795 by Miss Herschel in the constellation Cygnus, and with another seen in 1805. Its orbit is an ellipse of comparatively small dimensions, wholly within the orbit of Jupiter; its period is 1260 days, or about three years and three-tenths. It has been frequently observed since.
Another comet, the history of which is of the utmost importance in the latest theories regarding the connection of these bodies and the periodic showers of shooting-stars, is one known as Biela's comet, discovered in 1826. It revolved about the sun in about 6.75 years, and was identified as the same comet which was observed in 1772 and in 1806. Its returns were noted in 1832, 1839, and 1845. In 1846 it divided into two, returned double in 1852, but has not since been seen, the Supposition being that it has been dissipated, and that it was represented by a great shower of meteors that were seen in November 1872. One of the most remarkable comets of recent times was that known as Donati's, discovered by Dr. Donati of Florence in 1858. It was very brilliant in England in the autumn of that year, and on the 18th of October was near coming into collision with Venus, The year 1881 was remarkable for the number of comets recorded. During that year no fewer than seven comets, including the well-known short-period comet Encke's, were observed. Research Comet
Most British boroughs came into being through the action of the King or some great noble or bishop in selecting a strong point, primarily as a centre of defence, in late Anglo-Saxon or early Norman times. In the more peaceful days, the twelfth and thirteenth centuries, town burgesses began to increase their freedom to control markets and trade by purchasing charters, or documents setting out the town's right to the status of borough, free to conduct its own affairs in return for an annual payment to the King. The wording of the charter often included the right to hold a weekly market and an annual fair. The market was the most important weekly event in the life of a mediaeval town, and the essential nucleus of the town became the market square. This was the place where agricultural produce from the surrounding countryside could be sold, and where the town craftsmen could display their wares. Stalls and booths, at first temporary and later permanent, began to be erected in the centre of the market place, and outlying parts of the
market were set aside for the sale of livestock. Later, many towns acquired a market hall, or town hall, with a meeting hall for the transaction of business on the upper floor and open arches at ground level where goods might be displayed out of the rain. The market was concerned with supplying local needs; a similar form of business held in certain towns was the fair which had a wider significance because they attracted traders from other parts of England and even from the Continent. At fairs one might buy the specialised products of certain parts of England, such as Sussex iron, Worcestershiresalt, Derbyshire lead or Cornishtin, or spectacle lenses ground at Augsburg in Germany, beaten copperware from Dinant in modern Belgium or cutlery from Solingen in Germany. Research Market Towns
The Philadelphia Museum was the earliest public museum in the United States. It was founded by Charles Wilson Peale in 1785, and had as a nucleus a stuffed paddle-fish and the bones of a mammoth. Research Philadelphia Museum
Acellular is a biological term describing tissues or organisms that are not made up of separate cells, but often have more than one nucleus. Examples of
acellular structures are muscle fibres. Research Acellular
The diatomaceae are a large class of unicellular algae occurring in fresh and salt water and on damp soil. The cells are of diverse forms, their walls silicified and formed of two over-lapping valves, like a box and lid. The cell has a single, complex nucleus, brownish yellow chromatophores, and often contains much oil. The frond secretes a very large quantity of silex, which is formed in each cell into three portions: two generally symmetrical valves and a connecting hoop. The species consist of single free cells, or the cells remain attached so as to form linear, flabelliform, circular, or geniculate fronds, or in some cases the cells or frustules are inclosed in a transparent gelatinous sheath or frond. The ordinary method of increase is by cell division. Diatomaceae are found fossil, forming considerable deposits of tertiary age, as at Bilin, Richmond in the United States, etc. Fossil polishing powders, as tripoli and bergmehl, are composed of them. They are abundant in guano. Research Diatomaceae
Gregarinidae are a class of minute animal organisms comprising the lowest forms of Protozoa found parasitic in various animals, especially the cockroach and earthworm. The Gregarinidae consist of an outer colourless transparentmembrane, with only faint signs of a fibrillose structure inclosing a granular mass in which there is a nucleus surrounded by a clear space. Research Gregarinidae
The Iomud (also known as the Iomudskaya or Yamud) is a breed of Turkmenistanhorse originating from the ancient Turkmenian horses. It was developed by the
Iomud tribe in the Tashauz oasis in southern Turkmenia. As the breed occupied the margin of the Turkmenian breed area, it was influenced by steppe breeds. During the 14th century, it was further influenced by Arabian stallions. In contrast to the Akhal-Teke, the Iomud breed is kept in herds in the desert and semi-desert. The Iomud stands 15 hands high, has a large clean-cut head, sometimes Roman-nosed, a medium-long neck; medium-high withers, a solid back with small curve to the withers, nicely turned and regularly sloping croup, a shallow chest; clean fine legs, often bowed; a sparse mane and tail and delicate skin. The colour is grey or chestnut, rarely golden chestnut or black. The Iomud is a long-lived healthy horse. It shows soft 'floating' action. The purebred population has declined substantially. Stud farms to preserve the Iomud genotype were set up in Turkmenia in 1983. They are charged with protecting the breed and restoring the breedingnucleus to a size of 240-250 mares from the present 140 mares. A conservation farm is being established in
the Kyzyl-Atrek district. Research Iomud
Prokaryote or procaryote is the biological term for an organism in which the genetic material is not enclosed in a cellnucleus. Bacteria are
prokaryotes. It is believed that eukaryotic cells probably evolved as symbiotic associations of prokaryotes. Research Prokaryote
In biology, schizogony refers to a type of cell reproduction involving multiple fission; the nucleus divides many times and the nuclei are separated into daughter cells. Research Schizogony
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Comet
