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140 COMET Prussia. He subsequently repaired to Transyl- vania, and in 1650 elaborated rules for the Prot- estant college of Saros-Patak in Hungary. Re- turning to Lissa in 1654, he again lost all his books, manuscripts, and fortune by the Polish war of 1657, and spent the latter part of his life in Holland. As a writer in the Czech language he is highly esteemed for his classical style. As a school reformer he was the forerunner of Rousseau, Basedow, and Pestalozzi, suggested a mode of instruction which renders learning attractive by pictures and illustrations, and wrote the first pictorial school book, Orbis Sensualium Pictus (Nuremberg, 1658). For instruction in foreign languages he recom- mended combining with the teaching of the foreign words explanations of the ideas they express. His most celebrated works in this department, Janua Linguarum Reserrata (Lis- sa, 1631), and PansopJiicB Prodromus (Lissa, 1639), were translated into many languages. COMET (Gr. KOfifrw, long-haired), a celestial body presenting a nebulous aspect, and travel- ling under the sun's attraction. Many of these bodies are distinguished by a remarkable tail- like appendage. The greater number of those hitherto known have revolved round the sun on a path whose observed portion belonged to an exceedingly elongated ellipse, or was even parabolic or hyperbolic. A few, however, travel in closed orbits around the sun in known pe- riods. It has been supposed that some among the ancients suspected the periodic motions of the planets ; but the only evidence we have on the subject is vague and indefinite. Tycho Brahe was the first to prove by direct obser- vation that, comets are not mere phenomena of pur own atmosphere, but certainly further away than the moon. Newton, after estab- lishing the theory of gravitation, asserted that comets obey the laws of solar attraction, and therefore move either on elliptic, parabolic, or hyperbolic paths. From observations of the comet of 1680 (commonly called Newton's comet) Dorfel, a clergyman of Saxony, was led to the conclusion that the course of this object was parabolic. But the first real proof of the nature of cometary orbits was afforded by the researches of Halley into the motions of the comet of 1682 (Halley's comet). Halley computed the orbit of this comet, and having found that the figure of the orbit was either parabolic or a very extended ellipse, he exam- ined the records of ancient comets, and after incredible labor succeeded in discovering two whose motions agreed very closely with those of the comet of 1682. One had been observed by Appian in 1531, the other by Kepler in 1607; and Halley noticed that the intervals between the three years 1531, 1607, and 1682 are near enough to equality to suggest that one and the same comet had been observed on all three occasions. Finding that comets were ob- served in 1305, 1380, and 1456, he was further confirmed in the idea of the periodicity of this comet's returns ; and he was thus led to pre- dict the return of the comet about the end of 1758 or the beginning of 1759. He placed the return somewhat later than the former observ- ed intervals would have suggested, because he found that the attraction of Jupiter would re- tard the comet. When the time for its return approached, many eminent mathematicians re- computed the date of its perihelion passage, and Clairaut announced that this passage would occur between March 13 and May 13, 1759. The event actually took place on March 13, 1759 ; and it has been shown that a large part of the discrepancy between this date and the mean date of Clairaut's two months would have been removed had Clairaut known of the ex- istence of Uranus, and so taken the disturbing influence of that planet into account. On the next return of the comet in 1835, the epoch of perihelion passage was predicted much more accurately ; indeed, the actual event occurred within two or three days of the dates severally announced by Pontecoulant and Rosenberger. The observations of other comets have still further confirmed Newton's theory of cometic motions. All comets show a coma or haze of light. In nearly all cases there is a bright nucleus within this haze, and in a considerable number of instances, but not by any means in all, the comet shows a tail. When a large and complete comet, that is, a comet which pos- sesses a coma, nucleus, and tail, is approach- ing the sun, the haze of light usually changes from a rounded to an elongated figure. After- ward the comet's light presents a streaky or "combed out" appearance, and then presently a tail is thrown out on the side away from the sun. The tail usually grows longer and bright- er as the comet approaches the sun, and con- tinues in existence for some time after the comet has begun to pass away from the sun's neighborhood. But there is a considerable variety in this respect among different comets. Some which have shown beautiful tails as they neared the sun, have reappeared after the peri- helion passage with only a short tail or with- out any tail at all. Others which have shown only insignificant tails while approaching their perihelion, have "reappeared magnified and glorified, throwing out an immense tail and exhibiting every appearance of violent excite- ment." Most of the comets of short period are tailless or have tails barely discernible. An ex- amination of the drawings prepared for the third volume of the "Annals of the Observa- tory of Harvard College," to accompany the record of Prof. Bond's observations on Dona- ti's comet of 1858, will teach more respecting the actual processes of change which large comets undergo than any amount of verbal de- scription. It has been justly remarked by Sir John Herschel that these "engravings, in point of exquisite finish and beauty of delineation, leave far behind everything hitherto done in that department of astronomy." Among the comets most remarkable either for great splen- dor or enormous real dimensions in recent times