Extract of a Letter from Prof Encke to Mr. Airy. 137 



terwards fix upon this very + as the sign of addition ; and 



not less strange if after having established the latter in his 



own mind, he should have gradually drop; td into the former. 



I remain, Gentlemen, yours faitLtully, 



University College, Jan. 12, 1842. A. De Morgan. 



XXII. Ephemeris of the Periodical [Encke' s] Comet, 184'2; 

 ajid on tlie Mass of the Planet Mercury. By Professor 

 Encke, in an Extract from a Letter to Mr. Airy*. 



THE comet of short period comes to perihelion on the 

 12th of April next; and judging from its present course 

 and from former experience during Mr. Henderson's resi- 

 dence at the Cape of Good Hope, it may be well observed 

 there during the end of April and the whole of May, and pro- 

 bably also in June. May I then trouble you with the request 

 to get the accompanying ephemeris conveyed there, or to the 

 southern hemisphere generally; and also to provide for its 

 circulation in England ? I should think that, with the present 

 active communication between England and all parts of the 

 world, there is yet time enough to send the ephemeris to the 

 astronomer at the Cape of Good Hope by the end of March ; 

 and it would have the greater interest for him, because to this 

 time 710 return of the comet since 1819 has been missed. The 

 ephemeris is not strictly founded on all the earlier obsei'va- 

 tions, because it was impossible for me, notwithstanding all 

 my endeavours, to reduce completely the observations of the 

 comet made here in 1838. The compared stars still required 

 some more observations for their determination. Meanwhile, 

 I have provisionally determined a correction of the last ob- 

 servations of 1838, or rather a correction of the calculations 

 relating to that time, which will not be far from the truth. 

 Upon this provisional reduction, and the calculation of the 

 perturbations produced by Jupiter alone, the elements now 

 given for 1842 are founded. Judging, however, from earlier 

 experience, I believe that even with this incompk te calcula- 

 tion, the predicted place will be wrong by only about a few 

 minutes. The error certainly cannot amount to hfi1f a degree, 

 consequently the comet must be found, if it is really visible, 

 and if the search be made with care. 



The difference between the observation and the calculation 

 in the year 1838, is with great probability to be attributed to 

 a very important error in the hitherto received mass of Mer- 

 cury. 'J^liis mass is the same that Lagrange, in the Berlin 

 Mem. 1 782, has derived from a hypothesis on the density of 

 the planets, according to which the density ought to increase 

 * As circulated by the Astronomer Royal. 



