COMETS. 



66. On the 28th of February, 1846, M. de Vico discovered a 

 comet at Rome, whose orbit, calculated by MM. van Deinse 

 and Pierce, appears to be an ellipse, with a period of 72-73 

 years. 



67. A comet was discovered by M. Brorsen at Altona, on the 

 20th of July, 1847 ; the o*bit of which, calculated by M. d' Arrest, 

 appears to be an ellipse, with a period of 75 years. 



68. On the 27th of June, 1852, a comet was discovered by 

 M. Westphal at Gottingen, and was soon afterwards observed by 

 M. Peters at Constantinople. The calculation of its orbit proves 

 it to be an ellipse, with a period of about 70 years. 



69. A synoptical Table, presenting the data necessary to deter- 

 mine the motions of these six comets, may be seen by a reference 

 to the " Hand-Book of Astronomy." 



70. In fig. 5 (p. 145), is presented a plan of the orbits, brought 

 upon a common plane, and drawn according to the scale indicated. 

 This figure shows, in a manner sufficiently exact for the purposes 

 of illustration, the relative magnitudes and the forms of the six 

 orbits, as well as the directions of their several axes with relation 

 to that of the first point of Aries. 



71. By comparing the elements given in the table referred to 

 above, and the forms and magnitudes of the orbits shown in the 

 diagram, with those of the first group of elliptic comets given in 

 Table III, " Hand-Book of Astronomy," chap. XVIII, and drawn 

 in fig. 2 (p. 172), it will be perceived that the planetary charac- 

 teristics noticed in the latter group, are nearly effaced. Five of 

 the six comets composing the second group, revolve in the common 

 direction of the planets, and this is the only planetary character 

 observable among them. The inclinations, no longer limited to 

 those of the planetary orbits, range from 18 to 74. The eccen- 

 tricities are all so extreme, that the arc of the orbit near perihelion 

 approximates closely to the parabolic form, and the most remark- 

 able body of the group, the comet of Halley, revolves in a direction 

 contrary to the common motion of the planets. 



But it is more than all in the elongated oval form of their 

 orbits, that this group of comets differs, not only from the planets, 

 but from the first group. While their perihelia are at distances 

 from the sun, between those of Mars and Mercury, their aphelia 

 are from two to five hundred millions of miles outside the orbit 

 of Neptune. Thus, the comet of Halley, for example, in perihe- 

 lion, is at a distance from the sun less than that of Venus; but 

 at its aphelion, its distance exceeds that of Neptune by a space 

 greater than Jupiter's distance from the sun. The mean angular 

 motion of this comet is nearly the same as that of Uranus ; but 

 its angular motion in perihelion is three times that of Mercury ; 

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