On the CalcyJation of Parallax, 413 



and when she passes beyond these points, her apparent altitude, 

 instead of being diminished by the elUpticity, is actually in- 

 creased by it. 



The whole effect appears to be most conveniently comJ)uted 

 by considering the place of the apparent zenith as always 

 brought nearer to the pole than the true zenith, by a quantity 

 depending on the latitude only, and which is equal to ^Jg^ cos. 

 2 (lat.—45°)=:^J^ COS. 2 lat.-QO^rr^Jg. sin. 2 lat. or to 11' 14* 

 X sin. 2 lat. ; and the correction of the apparent altitude will 

 be obtained by multiplying this quantity into the cosine of the 

 azimuth. The computation being referred to this point instead 

 of the apparent zenith, it will be necessary to apply the same 

 correction, in the case of a lunar distance, to both the altitudes 

 concerned, according to the respective azimuths, by means of 

 the following rule : 



Azimuthal Correction of Parallax, 



Add together the logarithmic cosecant of twice the latitude, 

 the secant of the azimuth, and the constant logarithm 1.2048, 

 the sum will be the proportional logarithm of the correction cf 

 the altitude for the earth's ellipticity, to be added when the 

 moon or star is north of the east or west in the northern hemis- 

 phere, and to be subtracted when south, and the reverse in the 

 southern hemisphere. The altitudes so corrected will give the 

 true parallax, from the reduced equatorial parallax, by the ordi- 

 nary rules, and the refraction without sensible error. 



ii. Places of the Comet of 1822, computed by Professor Encke, 

 and communicated by Dr. Olbers. 



Professor Encke has considered the effects of Saturn, Jupiter, 

 Mars, the Earth, Venus and Mercury, on the Comet, throughout 

 the whole interval from 1786 to 1819; but he has found that 

 the attraction of Jupiter only will have any material effect on the 

 time of the next perihelium, in 1822; this effect, however, 

 being very considerable, since the least distance of the comet 



