COMETS. 



Nucleus. 78. Coma. 79. Origin of the name. 80. Magnitude of 

 the head. 81. Magnitude of the nucleus. 82. The tail. 83. Mass, 

 density, and volume of comets. 



THE appearance of the comet in 1456, was described by con- 

 temporary authorities to have been an object of " unheard-of 

 magnitude ; " it was accompanied by a tail of extraordinary length, 

 which extended over sixty degrees (a third of the heavens), and 

 continued to be seen during the whole of the month of June. The 

 influence which was attributed to this appearance, renders it pro- 

 bable that in the record there exists more or less of exaggeration. 

 It was considered as the celestial indication of the rapid success of 

 Mohammed II., who had taken Constantinople, and struck terror 

 into the whole Christian world. Pope Calixtus II. levelled the 

 thunders of the Church against the enemies of his faith, terres- 

 trial and celestial, and in the same bull exorcised the Turks and 

 the comet ; and in order that the memory of this manifestation of 

 his power should be for ever preserved, he ordained that the bells 

 of ail the churches should be rung at midday a custom which is 

 preserved in those countries to our times. It must be admitted 

 that, notwithstanding the terrors of the Church, the comet pur- 

 sued its course with as much ease and security, as those with which 

 Mohammed converted the church of St. Sophia into his principal 

 mosque. 



The extraordinary length and brilliancy which was ascribed to 

 the tail upon this occasion, have led astronomers to investigate 

 the circumstances under which its brightness and magnitude would 

 be the greatest possible ; and, upon tracing back the motion of 

 the comet to the year 1456, it has been found that it was then 

 actually under the circumstances of position with respect to the 

 earth and sun most favourable to magnitude and splendour. So 

 far, therefore, the results of astronomical calculation corroborate 

 the records of history. 



55. In the interval of three-quarters of a century* which elapsed 

 between the announcement of Halley's prediction and the date of 

 its expected fulfilment, great advances were made in mathematical 

 science ; new and improved methods of investigation and calcu- 

 lation were invented ; and, the theory of gravitation was pur- 

 sued with extraordinary activity and success through its conse- 

 quences in the mutual disturbances produced upon the motions 

 of the planets and satellites, by the attraction of their masses one 

 upon another. As the epoch of the expected return of the comet 

 to its perihelion approached therefore, the scientific world resolved 

 to divest, as far as possible, the prediction, of that vagueness 

 which necessarily attended it owing to the imperfect state of 

 science at the time it was made, and to calculate the exact effects 

 178 



