Professor Bessel on Meridional Instruments. 227 



Puppis and Vela ; and substituting the term Malus for Pixis Nau- 

 tica, since it contains four of Ptolemy's stars that are placed by hiin 

 in the mast of the ship. 



6. That the original constellation Argo, on account of its great 

 magnitude and the subdivisions here proposed, be carefully revised 

 in respect of lettering, in the following manner : — First, in order to 

 preserve the present nomenclature of the principal stars, all the stars 

 in Argo (that is, in the general constellation, regarded as including 

 the subdivisions above-mentioned) indicated by Greek letters, by 

 Lacaille, to be retained, with their present lettering, under the ge- 

 neral name Argo. Secondly, all the remaining stars, to be desig- 

 nated by that portion of the ship in which they occur, such as Carina, 

 Puppis, Vela and Malus, and to be indicated by the Roman letters 

 adopted by Lacaille, as far as the fifth magnitude inclusive. And no 

 two stars, far distant from each other in the same subdivision, to be 

 indicated by the same letter ; but, in cases of conflict, the greater 

 magnitude is to be preferred ; and, when they are equal, the pre- 

 ceding star to be fixed upon. 



7. That the constellations, which Lacaille has designated by two 

 words, be expressed by only one of such words. Thus, it is pro- 

 posed that the several constellations indicated by Lacaille as Appa- 

 ratus Sculptoris, Mons Mensae, Coelum Scalptorium, Equuleus Pic- 

 torius, Piscis Volans, and Antlia Pneumatica, be called by the re- 

 spective titles of Sculptor, Mensa, Coelum, Pictor, Volans and Antlia; 

 contractions which have on some occasions been partially used by 

 Lacaille himself, and are very convenient in a registry of stars. 



VIL Extract (translated) from a Letter of Professor Bessel to Sir 

 J. F. W. Herschel, Bart., dated Konigsberg, January 22, 1844. 

 Communicated by Sir John Herschel. 



" I think it may be interesting to you, or to one or other of your 

 astronomical friends who occupy themselves with meridional instru- 

 ments, to be made acquainted with a result which I have obtained 

 from a theoretical investigation, the object of which was to determine 

 the eflFect of gravity upon the figure, and consequently on the divi- 

 sions, of a circle fixed in the vertical plane. The effect will mani- 

 festly be, that the radii in the upper part of the circle will be short- 

 ened ; those in the lower part lengthened ; and all, with the excep- 

 tion of the vertical ones, bent downwards. These changes, the mag- 

 nitude and law of which must depend on the special construction of 

 each circle, in two instances known to me (namely, those of two 

 meridian instruments by Repsold, each of which is furnished with 

 two circles at opposite ends of the axis, each being read by four mi- 

 croscopes), have become prominently sensible ; giving rise to this 

 ejBfect, namely, that on turning the instrument through 180°, a dif- 

 ferent measure is given by its two circles. From this result it fol- 

 lows that the supposition of the amount of change being insensible, 

 or of the influence of gravity on the circle being eliminated by the 

 four readings, is without foundation ; and that there is, therefore, 

 cause for ajjprehending in general that every circle gives an erro- 

 neous measure of the zenith-distances, and erroneous to an extent 



Q2 



