IJH.] Imperial Institute of Franoe. 309 



Burkhardt, founded upon a greater number of observations, and 

 upon newer researcbes, offers in consequence a greater degree of 

 probability. A reason of great weigbt adds strengtb to tbe neces- 

 sity of diminisbing tbe mass of matter of Venus, and tbis reason 

 long excited distrust in tbe autbor of tbe tables. In wbatevef 

 manner be combined tbe observations of Lacaille, Mayer, Bradley, 

 Le Gentil, Maskelvnc, Piazzi, and bis own, be could never ob- 

 tain more tban 48"' for tbe secular variation of tlie obliquity of tbe 

 ecliptic. The mean obliquity whicb be found in 1800 has been 

 since confirmed by all tbe solstices observed at Paris. Tbat which 

 results for IJoO, from so many observations agreeing remarkably 

 well with each other, caimot be wrong further tban \". Hence he 

 concludes, tbat the secular diminution cannot be greater than 50". 

 He has never believed tbat it amounted to 52". We may therefore 

 suppose to Venus a quantity of matter which would give 'IS" or 50" 

 for the secular diminution, and give to tbe equation of Venus in the 

 solar tables, tbe value which will result from this supposition. 



:\I. Burkhardt proposes a diminution of 1" for tbe greatest equa- 

 tion of tbe centre. If we collect together all these corrections, 

 it will follow, tbat tbe sun's place, calculated at present for 1850, 

 jmay differ 6" from a perfect observation made at that time. But 

 before this can happen we must suppose what is scarcely possible, that 

 the three errors are all a maximum, and have the same sign. Tbe 

 greatest error tbat can be sujjposed is 3" at the end of 50 years. 

 We wish the instruments by tbat time may be brought to such per- 

 fection that an astronomer can make a single observation with no 

 greater an error. But this sliglit error may be easily avoided by 

 adopting tbe corrections of IM. Burckhardt. The most important 

 of theni is that of the secular motion. It is easy to diminish^ it 

 ^^ of a second per year. Tlu' two otlier corrections would give 

 stUl less trouble; but as they aie periodic, and often almost nothing, 

 they may in many cases be neglected. 



Corrections, so little sensible as not to pass the limits of the 

 errors of the best observations, may pass for a confirmation of the 

 tables, as well as for an amelioration of them. We may be even 

 sorry for astronomers devoting themselves to calculations so long 

 and so fastidious, and yet obtaining only results so little different 

 from those which we jmssessed before. l'>ut tbe tablqs of the sun 

 constitute the foundation of all our calculations : they cannot be 

 too frequently verified. It is particularly the duty of the members 

 of the Board of longitude to attend to this verification. It was on 

 this account that M. Burckhardt lias undertaketi a still more labo- 

 rious investigation of tbe tables of the moon, in order to obtain 

 ameli(»rutioiis of the same kind. 'I'he very smalluess of these cor- 

 rections is a most satisfactory proof of the singular perfection which 

 sjitronoinical observations and calculations have reached. 



( Tv it .cti»timitd.) 



