the celebrated Comet o/1264- and 1556. 51 



idea of the path followed by the comet, and we have ample 

 information for a rough determination of the elements. 



When Halley published his Synopsis of Cometary Astro- 

 nomy, he gave a set of parabolic elements for the comet of 

 J 556, founded upon the observations made by Paul Fabricius; 

 but he remarks that these elements are not so certain as those 

 of other comets he had computed, the observations being made 

 "neither with sufficient instruments nor due care," and by no 

 means to be reconciled with any regular calculation. 



The elements of the comet of 1264 were first computed by 

 Mr. Dunthorne. His discussion of the observations and cir- 

 cumstances relating to the comet's apparition are published in 

 vol. xlvii. of the Philosophical Transactions. The elements 

 are chiefly founded on the £Puthority of a manuscript preserved 

 in the library of Pembroke Hall College, Cambridge, entitled 

 Tractatusfratris JEgidii de Cometis. But it must be observed 

 there are manifest contradictions in this account not easily set 

 right. The other authorities consulted were the Chronicon 

 Sampettinum Erphiirtense and the Chronicle of John Vitodu- 

 ranus. The orbit deduced by Mr. Dunthorne much resem- 

 bles that calculated by Halley for the comet of 1556. 



In the Memoirs of the Royal Academy of Sciences at Paris 

 for 1760, appears a valuable memoir by M. Pingre on the 

 comet of 1264. After collecting together a great number of 

 accounts from different chronicles and histories of the day, 

 he proceeds to the discussion of the elements. The contra- 

 diction in the Cambridge manuscript which relates to the 

 comet's motion in longitude is pointed out; and since this 

 manuscript was Mr. Dunthorne's chief authority, it might be 

 supposed that his orbit would differ entirely from M. Pingre's. 

 This, however, was not the case ; for although there are dif^ 

 ferences of some moment in one or two of the elements, there 

 is still a striking similarity between the two orbits taken as a 

 whole, and M. Pingre's approaches much nearer than Mr. 

 Dunthorne's to the orbit of the comet of 1556. A closer 

 agreement might have been produced if he had not wished to 

 preserve the path laid down by Thierri de Vaucouleurs with 

 as little alteration as possible. M. Pingre concludes from his 

 researches that there is little doubt of the identity of the comets 

 of 1264 and 1556, and, therefore, that the return to perihelion 

 may be expected to take place in the year 1848. In No. 493 

 of the Astronomische Nachrichten will be found the results of 

 my first calculations relating to this comet. I have there de- 

 duced elements from the observations by Fabricius in 1556, 

 and computed an ephemeris for comparison with the comet's 

 observed path. The agreement, though not so close as could 

 be wished, was the best that could be obtained from the data 



E2 



