Observations on Comet II. 207 



have agreed among themselves that it was "not worth the 

 trouble of looking after." May we arrest some such party of 

 observers, and ask them to pursue the subject of comets with 

 us during a few pages ? till perhaps the general interest of the 

 theme may lead them to reconsider their verdict, and welcome 

 every comet for the opportunities which it affords for the eluci- 

 dation of problems interesting from their very difficulty. 



The comet which we have just seen, has come, as all con- 

 spicuous comets, with one remarkable exception — Halley's 

 Comet — have come, unexpectedly, and unforetold. It was dis- 

 covered, independently, by at least three observers,* about the 

 third week in July, as a faint, hazy comet, discernible only in 

 the telescope. In the same manner, Comet Y. of 1858, better 

 known as "Donati's Comet," was discovered. Others have 

 escaped notice till detected with the naked eye. No sooner 

 however is a comet seen, than the work of prediction begins. 

 The comet is subjected to a most rigorous inquiry. The 

 elements of its orbit are roughly calculated, and improved as 

 observations accumulate, by a multitude of ardent and expert 

 computers. Old records are ransacked, and old observations 

 put into tangible shape, so as to rescue from oblivion the orbits 

 of ancient comets which present any similarity to that of the 

 new visitor. f Then, the comet's probable changes of apparent 

 position and brightness for several weeks to come are estimated, 

 with a degree of minuteness which might well lead the unin- 

 structed to suppose that the stranger must have been con- 

 fidently expected for a long period. 



Halley's, as we have said, is the one conspicuous comet 

 which was really expected. Its appearance in 1758 had been 

 foretold by Halley many years before. It came punctually, 

 and was again promised for the year 1835. Those who re- 

 member the announcement of its approach in the almanacs 

 which came out at the end of the year 1834, and who subse- 

 quently saw the comet in the following October, are likely to 

 have obtained a strong impression of the degree of regularity 

 to be found among the movements of these strange wanderers. 

 Nor is Halley's the only comet which has been observed to 

 return, although no other visible to the naked eye has done so. 

 There are six comets, visible in the telescope, and known as 

 the " comets of short period," which have several times re- 

 turned at the calculated dates, and again retreated from view. 

 They are remarkable for the smallness of their orbits which, 

 notwithstanding their elliptical shape, are entirely included in 



* These were Mr. "Tuttle, at Cambridge, in America, on July 18th, MM. 

 Toussaint and Pacinotti at Florence, on July 22nd, and M. Eosa at Rome on 

 July 25th. 



t Herschel's Outlines of Astronomy, art. 597. 



