COMBT 



Ecdesias in Unitntf r'nttrtuii Itolii'iimniin (1032; 

 republished 1702); and his J'umiophiie Prodrotmu 

 ( 1 i\'M), an attempt at a complete statement of the 

 4-irrlenf knowledge. In 1041 Comeniuswas invited to 

 England liy parliament, through the philanthropist 

 llartlieb, to a-sist 111 reforming tlie system of public 

 iiiMnii-iiun : Inn as the l>n-;ikiir_' uut of the Civil 

 War prevented tlie execution of this design, he went 

 tn Sweden (lt>42). There lie was patronised by 

 Oxenstieru, who gave him a commission to draw up 

 a plan for the organisation of schools in Sweden ; 

 ami this lie completed at Elbing, four years after- 

 wards. He also put to press (1643) a treatise on 

 Pansophia. He returned to his Polish home at 

 Lissa 111 1048, and elaborated his work there. He 

 next went (1050) to Saros-Patak, Hungary, for a 

 .similar purpose. Here he composed his celebrated 

 (h-ln'fi Sfnuiin/iinii Pictnn, or The Visible World 

 (Xurem. 1058), the first picture-book for children, 

 which has been often reprinted and imitated. 

 Finally, he settled in Amsterdam, where he pub- 

 lished several other works. He died at Naarden 

 on the 15th November 1671. Bacon's speculations 

 appear to have stirred the imagination or Comenius 

 to the conception of universal and systematised 

 learning, to which he gave the name of Pansophia 

 or Encyclopaedia. 



His educational and pansophic works were pub- 

 lished at Amsterdam (4 vols. 1657), and dedicated 

 to the city of Amsterdam in recognition of his hos- 

 pitable treatment there. In education he was a 

 realist ; he was also fervently evangelical, and his 

 whole system was intended to lead up to know- 

 ledge, virtue, and piety. Late in life a mystical 

 tendency was apparent in his works. Whatever 

 may be thought or his educational system, he first 

 folly developed educational method, made import- 

 ant reforms in the teaching of languages, and 

 introduced into schools the study of Nature. See 

 Laurie's Conien ins, /i is Life ana Work (1881), a 

 (.iTinan Life by Kvacsala (1892), and Keatinge's 

 translation of The Great Didactic ( 1890). 



Comet* The word comet is derived from the 

 Greek koine, ' hair,' a title which had its origin in 

 the hairy appearance often exhibited by the haze 

 or luminous vapour, the presence of which is at 

 first sight the most striking characteristic of the 

 celestial bodies called by this name. The general 

 features of a comet are a definite point or nucleus, 

 a nebulous light surrounding the nucleus, and a 

 luminous train preceding or following the nucleus. 

 Anciently, when the train preceded the nucleus 

 as is the case when a comet has passed its peri- 

 helion, and recedes from the sun it was called the 

 beard, being only termed the tail when seen follow- 

 ing the nucleus as the sun is approached. This 

 distinction has disappeared from all modern astro- 

 nomical works, and the name tail is given to 

 the appendage, whatever its apparent position. 

 Neither the tail nor the nucleus is now considered 

 an essential cometary element, but all bodies 

 visible in interplanetary space, which describe 

 <nKits round the sun of an extremely elongated 

 form, are classed as comets. There are several 

 plain points of difference between comets and 

 planets. The planets move all in the same 

 direction, from west to east, which is astronomically 

 called 'direct motion;' but the movements of 

 comets art- often from east to west, or retrograde. 

 The orbits of all the planets are confined to a /one 

 of no great breadth on either side of the ecliptic : 

 but the paths of comets cut the ecliptic in every 

 direction, some being even perpendicular to it. 

 Tlii> orbits of all the planets are nearly circular; 

 or, more properly speaking, are ellipses of very 

 small eccentricity. The orbits of comets, on the 

 otln-r hand, pn-sent every variety of eccentricity, 

 some of them being ellipses or elongated closed 



COMET 



375 



orbits of various degree* of elongation ; other*, 

 hyperbola** ; while tlie majority have a form of 

 orbit not diliering sensibly from the parabola, 

 which is the limiting form of curve to which both 

 the ellipse and hyperbola approximate under given 

 condition-. 



Let p be the point of perihelion pannage of a 

 comet, and let the direction of it - motion be in the 

 direction indicated by the arrow. There is a 

 certain velocity of motion at this point which 

 would give the orbit the form of a parabola, /"/', 

 the direction of motion always tending to parallel- 

 ism with the straight line through jb. Any 



velocity less than this would cause it to describe a 

 closed curve, pa-a z , any greater velocity would 

 cause it to describe the hyperbola, pa*, m which 

 case it would approximate to the direction of the 

 straight line, CD, and would never return. This 

 would be the case if there were no disturbing force 

 to interfere with the sun's attraction ; and con- 

 versely, comets approaching the sun along the 

 various paths above described would pass the 

 perihelion with the various velocities above indi- 

 cated. Any attraction, however, of an extraneous 

 body interfering with the attraction of the sun 

 might change trie orbit from the ellipse to the 

 hyperbola, and vice versa, or from the parabola to 

 either. As, however, there is only one parabola 

 corresponding to infinite sets of ellipses and hyper- 

 Itolas, an interfering cause is not likely to change 

 the orbit from an ellipse or hyperbola to the para- 

 bolic form. 



There are twenty kiioini periodical comets, eleven 

 of which have been observed at more than one 

 perihelion passage. These latter have periods in 

 years as follows: Encke's, 3 '3; AVinnecke's, 5'6 ; 

 Brorsen's, 5'5 ; Tem pel's I, 6 ; D'Arrest's, 6'4 ; 

 Biela's, 6'6; Faye's, 7'4; Tuttle's, 13'8 ; Halley's, 

 76; Temper's II," 5'2; Swift's, 5'5. 



The orbits of forty-three comets appear to be 

 ellipses. These are probably periodical. The paths 

 of alxmt two hundred cannot be distinguished from 

 paral>olas. These are possibly periodical, as the 

 ellipse, when very eccentric, cannot near perihelion 

 be distinguished from a parabola. Only six or 

 seven have orbits possibly hyperbolic. 



Elliptic comets in a large* majority move directly 

 i.e. from west to east. Parabolic comets usually 

 move in the contrary direction. But, on the whole, 

 there is no general tendency in these bodies to 

 move either way. Periodic comets have orbits but 

 little inclined to the ecliptic ; but, on the whole, 

 cometary orbits show a tendency to gather round 

 a plane 'inclined 50 to the ecliptic. The perihelia 

 gather about two opposite regions, in longitude 

 60 to 120, and 240 to 300. The nodes of the 

 orl'its gather also about two regions, not, however, 

 directly opposite. 



The 'discovery that comets are celestial bodies, 

 extraneous to our atmosphere, is due to Tycho 

 Brahe, who measured the parallax of the comet 



