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
Envelopes are the paper covers that inclose letters or notes. They became common shortly after the introduction of the penny postage system in 1840 and were at first made chiefly by hand, but since the end of the 19th century were not only shaped, but folded, gummed, etc, by machinery. Envelopes for letters were first mentioned by Jonathan Swift in 1726. Research Envelope
Calamus or Sweet Flag (Acorus calamus) is a perennialherb of the order Araceae native to Europe, Asia and North America. It has narrow sword-shaped leaves and a thick branched rhizome. The flowering stem is also sword-shaped, and ends in a flattened envelope (apathe) from which emerges the flower-spike with hundreds of yellow-green coloured simple flowers. When crushed the leaves emit an odour of tangerine. Research Calamus
In popular language, a flower is the blossom of a plant, consisting chiefly of delicate and gaily-coloured leaves or petals; in botany, the term is restricted to the organs of reproduction in a phenogamous plant,.
A complete flower consists of stamens and pistils together with two sets of leaves which surround and protect them, the calyx and corolla. The stamens and pistils are the essential organs of the flower. They occupy two circles or rows, the one within the other, the stamens being in the outer row.
The stamens consist of a stalk or filament supporting a roundish body, the anther, which is filled with a powdery substance called the pollen.
The pistil consists of a closed cell or ovary at the base, containing ovules, and covered by a style which terminates in the stigma.
These organs are surrounded by the corolla and calyx, which together are called the floral envelope, or when they both display rich colouring the perianth. The leaves of the corolla are called petals, and those of the calyx sepals.
Some flowers lack the floral envelope, and are called achlamydeous; others have the calyx but are without the corolla, and are called monochlamydeous.
Flowers are generally bisexual, but some plants have unisexual flowers; that is, the pistils are in one flower and the stamens in another. Research Flower More pictures of Flower
Fruit is a botanical term for the mature ovary of a plant comprised of two parts, the pericarp and the seed. In simple terms, a fruit is the product of a plant comprising its seed or seeds and the envelope around the seed or seeds.
The newt (Molge) is a genus of Urodela. Newts are scaleless, lizard-like, semi-aquatic creatures related to frogs, but which keep their tales, that always resort to ponds during the breeding season. The males frequently differ from the females in the possession of high crests and other nuptial ornaments. All newts lay eggs, usually singularly, and these are attached to the leaves of submerged plants. By the skilful use of her feet, the female twists or folds the leaf around the egg, the eggs sticky envelope causing the leaf to stick to it, thus concealing the egg from predators. Infant newts are born with external gills, their transformation into lung-breathing newts taking place three or four months after hatching. Generally, after breeding newts leave the ponds and live in holes in the ground in the surrounding area.
The largest British newt is the Great Crested Newt also known as the Warty Newt so called on account of its warty skin. The Great Crested Newt is about 15 cm long, dark brown along the back with a bright orange underside marked with black spots. The male Great Crested Newt has a crest that runs from the head to the tail. Research Newt
In botany, the perianth is the outer floral envelope - calyx and corolla - which surrounds and to some extent protects the essential organs of generation in a flower. As a rule it consists of an outer layer of greenish parts called sepals, and an inner layer of brightly coloured parts called petals; but sometimes one or other of these layers is missing, and sometimes though they are both present, they are indistinguishable from each other. Many plants, especially trees, contain no trace of a perianth in their flowers, and these are usually wind propagated. Research Perianth
Spider (Araneidae) is an order of animals of the class Arachnida, all having eight legs. Most spiders are terrestial, but some live in fresh water.
The spiders have a body that is divided into two parts: the head and breast, fused into one piece; and the abdomen, usually all in one piece, and only in rare cases with hints of segmentation. Between these two parts there is typically a narrow waist. The region corresponding to the head bears two pairs of mouth parts: a pair of two-jointed poison-jaws or chelicerae; and a pair of sensitive, usually six-jointed pedipalps.
All spiders have a poisonous bite, but the bite is not severe except in a few tropical forms. The poison of the bird-catching spider (Mygale) kills a bird in a few minutes. In male spiders the tip of the pedipalp is complicated, it becomes a reservoir for spermatozoa at the mating season, and is used to transfer them into the female, where they fertilise the eggs just prior to the eggs being laid. In the main the pedipalps are organs of touch, with very sensitive tactile qualities.
On the top of the head are several pairs of simple short-sighted eyes. From the region corresponding to the thorax there arise four pairs of seven-jointed legs, ending in minute curved claws, by means of which spiders grip the surface on which they creep. At the end of the abdomen there are between four and six minute appendages transformed into spinnerets, from which the silken threads emerge. Each spinneret resembles the rose of a watering-can, and contains numerous minute tubes known as spinning-spools through which the silk issues. There may be hundreds of these spinning-spools and each is connected with an internal gland which produces the silk. The gland is enclosed in a muscular envelope, the contraction of which acting like a syringe, forces the liquid silk down a duct and out at the spinning-spool. There are sometimes three kinds of glands, producing different kinds of silk, and it rests with the spider to use more or fewer at one time, thereby adjusting the thickness of the thread produced. The thread is a fusion of many jets of liquid silk, which solidifies instantly it is exposed to the air.
A small minority of spiders breathe by two pairs of lung-books; all the rest breathe by two lung-books and by two or four tracheae like those of insects. The air enters the compartments of the lung-books through an external slit flush with the skin. In the partitions between the compartments of the lung-books the blood circulates and is purified. Research Spider
In fortifications an envelope is a work of earth, in the form of a single parapet or of a small rampart. It was sometimes raised in the ditch and sometimes beyond it. Research Envelope
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
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