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
The Gulf Stream is a well-known oceanic current, so called because it issues from the Gulf of Mexico. It owes its origin to the fact that the westward moving waters of the tropical portion of the Atlantic Ocean, encountering the eastward projection of South America, become divided into two currents, one setting southwards along the Brazilian coast, and the other northward past the mouths of the Amazon and Orinoco, into the Caribbean Sea. It then enters the Gulf of Mexico, and thence emerges through the Channel of Florida as the Gulf Stream. Its course is next to the north and eastwards, in a direction parallel to the coast of the United States, past Cape Hatteras, along the southern edge of the 'great banks' of Nantucket and Newfoundland (between the meridians of 48 and 60 degrees west), after which its course as a distinct current is less obvious.
In the earlier part of its course, especially when rounding the extremity of Florida, the Gulf Stream forms a well-defined current, distinguished by its high temperature and its deep blue or indigo colour. On account of the descent of the Polar or Baffin Bay current along the coast in a direction opposite to that of the Gulf Stream, the water on its inland side is colder than that to the eastward of it. The difference of temperature between the Gulf Stream and this cold current sometimes amounts to 20 or even 30 degrees Fahrenheit
The velocity of the Gulf Stream varies with its course. Within the Florida Channel it attains a mean of 65 miles per day, this sinks to 56 miles off Charleston, becomes 36 miles to 46 off Nautucket, and 28 miles to the south of the Newfoundland Banks; 300 miles to the eastward of Newfoundland its movement is hardly perceptible. At the bottom of the Florida Channel the observed temperature is 34 degrees that of the surface from 80 to 84 degrees Fahrenheit. Research Gulf Stream
A railway is a road made by placing on the ground on a specially prepared track, continuous parallel lines of iron or steel rails, on which carriages with flanged wheels are run with little friction and consequently at high velocity. The necessity for railways originated in the requirements of the coal-traffic of Northumberlandshire, where the first railways were constructed. In 1676 near Newcastle the coals were conveyed from the mines to the banks of the river by laying rails of timber straight and parallel; and bulky carts were made, with four rollers fitting those rails, whereby the carriage was made so easy that one horse could draw four or five chaldrons of coal.
The first railway (railroad) constructed in America was projected by Gridley Bryant in 1825, and extended from Quincy, Massachusetts, to the nearest tide-water. It was four miles long. The second railway extended from mines near Mauch Chunk, Pennsylvania to the Lehigh River. It was begun in 1827. Stephenson's locomotive came into use in 1829, and by 1830 there were twenty-three
miles of railway completed in the United States.
The Cursores, or Runners, are an order of birds, which includes the ostrich, rhea, emu, cassowary, and apteryx. The birds of this order are distinguished by their remarkable velocity in running, the rudimentary character of their wings, which are too short to be of use for flight, and by the length and strength of their legs. The breast-bone is destitute of the ridge or keel which it possesses in most birds, hence the name Ratitoe (from the Latin ratis, a raft). Research Cursores
Flying fish is a name for several species of bony fishes in which the pectoral fins are lengthened and wing-like and serve to sustain the fish in its short flights through the air. Generally, however, the name is limited to the species of the genus Exocoetus, which belongs to the family Scomberesocidae (mackerel-pikes). The pectoral tins, which are very large, are the principal instruments in their flight, serving to sustain the fish temporarily in the air after it has acquired an initial velocity in its rush through the water. After taking off the flying fish flex their tail from side to side to provide extra propulsion. Flying fish can pass through the air to a considerable distance, sometimes as much as 180 metres, which it does to escape from the attacks of other fishes, especially the dolphin. It is most common between the tropics. The best-known species are Exocoetus volitans, abundant in the warmer parts of the Atlantic, and Exocoetus exiliens of the Mediterranean. Some naturalists subdivide the genus into several, characterized by the presence or absence of barbels. Flying fish lay their eggs on floating debris, spawning in large groups and producing masses of eggs which can sink small boats if they remain stationary for several hours. Research Flying Fish
George Attwood F.R.S. was an English mathematician. He was born in 1745 and died in 1807. He is best known by his invention, called after him Attwood's Machine, for verifying the laws of falling bodies. It consists essentially of a freely moving pulley over which runs a fine cord with two equal weights suspended from the ends. A small additional weight is laid upon one of them, causing it to descend with uniformacceleration. Means are provided by which the added weight can be removed at any point of the descent, thus allowing the motion to continue from this point onward with uniformvelocity. Research George Attwood
Sir Richard Arkwright was an English inventor. He was born in 1732 at Preston, Lancashire and died in 1792. The youngest of thirteen children, he was a barber by trade, while travelling the country dealing in hair for wigs he became interested in the slow and clumsy processes used for spinning and weavingcotton, and when about thirty-five years of age he gave himself up exclusively to the subject of inventions for spinningcotton. The thread spun by Hargreaves' jenny could not be used except as weft, being destitute of the firmness or hardness required in the longitudinal threads or warp. But Richard Arkwright supplied this deficiency by the invention of the spinning-frame, which spins a vast number of threads of any degree of fineness and hardness, leaving the operator merely to feed the machine with cotton and to join the threads when they happen to break.
His invention introduced the system of spinning by rollers, the carding, or roving as it is technically termed (that is, the soft, loose strip of cotton), passing through one pair of rollers, and being received by a second pair, which are made to revolve with (as the case may be) three, four, or five times the velocity of the first pair. By this contrivance the roving is drawn out into a thread of the desired degree of tenuity and hardness. His inventions being brought into a pretty advanced state, Richard Arkwright removed to Nottingham in 1768 in order to avoid the attacks of the same lawless rabble that had driven Hargreaves out of Lancashire. Here his operations were at first greatly fettered by a want of capital; but two gentlemen of means having entered into partnership with him, the necessary funds were obtained, and Richard Arkwright erected his first mill, which was driven by horses, at Nottingham, and took out a patent for spinning by rollers in 1769. As the mode of working the machinery by horse-power was found too expensive he built a second factory on a much larger scale at Cromford, in Derbyshire, in 1771, the machinery of which was turned by a water-wheel. Having made several additional discoveries and improvements in the processes of carding, roving, and spinning, he took out a fresh patent for the whole in 1775, and thus completed a series of the most ingenious and complicated machinery. Notwithstanding a series of lawsuits in defence of his patent rights, and the destruction of his property by mobs, he amassed a large fortune. He was knighted by George III in 1786. Research Richard Arkwright
A sky marshal is a plain-clothed armed guard placed on an aircraft to defend against hijackers. Sky marshals first originated in the USA and were quickly adopted by the Israeli airline El Al. Typically a sky marshal is armed with a low-velocity semi-automatic pistol which will not puncture the aeroplane's fuselage when discharged. Research Sky Marshal
The Heckler and Koch HK53 is a compact German delayed blowback selective fire assaultrifle or sub-machine-gun. The HK53 is chambered for the 5.56 mm NATOcartridge and takes a 25 or 30-round magazine. It has a cyclic rate of 700 rounds-per-minute automatic with three-round burst and semi-automatic single shot operations selectable, and a muzzle velocity of 735 metres per second and an effective range of 400 metres. The Heckler and Koch HK53 has a retractable butt and a 225 mm long barrel. It is fitted with a post foresight and a rearsight comprising a V battle sight at 100 metres with apertures for 200, 300 and 400 metres. Research Heckler and Koch HK53
The Valmet M78 is a Finnish automatic assaultrifle produced from 1978 to 1986. It takes a 7.62 mm round from a 15 or 30-round box. It has a muzzle velocity of 718 meters-per-second and is sighted to 800 metres. It has a cyclic rate of 650 rounds-per-minute. Research Valmet M78
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Comet
