220 POOR 



defined head, or nucleus, of about 80" diameter and a tail which 

 reached its greatest length on July i, when it extended 2° 23'. 

 On July 4 the comet disappeared in the rays of the sun, not 

 to reappear until August 4, after which it could be seen by 

 the unaided eye until the 26th of August and in the telescope 

 until October 3, when its distance from the earth and sun be- 

 came so great as to render it invisible with the instruments of 

 that day. 



Parabolic elements were calculated by Pingre, Lambert and 

 others. These, however, did not satisfy all the observations and 

 it remained for Lexell to prove that the comet was travelling in 

 an elliptic orbit of 5.6 years' period. Lexell further showed, in 

 answer to various objections to the theory of elliptic motion, 

 that in May, 1767, the comet had passed very close to Jupiter 

 and that then its orbit had probably been greatly changed — 

 changed sufficiently to render it visible in 1770. He further 

 predicted a second approach of the comet to Jupiter in August, 

 1779, and stated that the occurrence might prevent the return 

 of the comet in 1781 or 1782. The comet was sought vainly 

 in those years, and has never been observed since. 



In 1 844 LeVerrier ^ presented to the Academic des Sciences 

 his researches upon the motion of this comet and upon the char- 

 acter of the disturbances by Jupiter m 1779. This paper is a 

 complete treatise upon the subject, discussing every phase of the 

 matter in a most thorough manner. Unfortunately, however, 

 LeVerrier found that the old observations were so crude that a 

 definitive determination of the orbit was impossible. A compar- 

 ison of the observed and computed places showed errors in the 

 original observations amounting in many instances to ten or even 

 fifteen minutes of arc. Unable to determine the exact elements 

 of the orbit, LeVerrier expressed the elements which best repre- 

 sented the motion of the comet in terms of an unknown and inde- 

 terminate quantity, /i. This quantity is carried through all the 

 calculations and the paper concludes with tables showing a num- 

 ber of possible orbits of the comet after its great disturbance by 

 Jupiter in 1779. 



' " Theorie de la Comete Periodique de 1770," Anna's de V Observatoirc tie 

 Paris, Tom. ill, p. 203. 



