1813.] the Rev, Nevil Maskelyne, 40S 



regulated at Greenwich by Bradley, and which had been trans- 

 ported with the greatest possible care, determined the number 

 of oscillations which it made less in St. Helena than at London, 

 in order to deduce from that observation the diminution of 

 gravity. 



The secondary\)bject of the voyage, the parallax of Sirius, 

 likewise failed ; but it produced an observation both curious and 

 useful. To know if Sirius had a sensible parallax, it was neces- 

 sary to have a more perfect instrument than that of LaCaille ; 

 it was necessary to observe the star in peculiar situations. The 

 first of these requisites depended upon the artist, the second 

 upon the astronomer. The Royal Society had got a sector made 

 on purpose, which was only fioislied just when the vessel sailed^ 

 and could not be verified at Greenwich. What was the surprise 

 of Maskelyne when he found that this instrument, destined for 

 the most delicate researches, gave him from one day to another 

 differences of 10'', 20'', and even 30", in the measure of the 

 same angle. In examining with care what could be the cause of 

 these singular variations, he discovered it without difficultyj 

 made himself certain of it by various proofs, and endeavoured to 

 correct it, but could succeed only imperfectly. He reduced the 

 error to 3", which was far from being sufficient for the object 

 that he had in view.* This obliged him to renounce his second 

 project. The result, however, was an amelioration in the con- 

 struction of these astronomical instruments. 



* This fault was occasioned by the plumb-line forming at its Upper extremity a 

 buckle by which it was fixed to a cylinder ^V^^ of an inch in diameter, placed 

 at the centre of the sector. It was impossible to direct the telescope to a star 

 without giving the cylinder a movement of rotation equal to the zenith dis- 

 tance of the star. During this movement, from the effect of adherence, the 

 cylinder displaced the line from its primitive position. Hence the arc which 

 had passed under the line was not the true distance of the star from the zenith. 

 Maskelyne filed down the cylinder to y^th of a line, and then the error was 

 reduced to 3". It was doubtless upon this occasion that the present mode of 

 suspension was thought of, M'hich consists in fixing the line higher up to a 

 point from which it may hang freely opposite to a point marked on the ante- 

 rior surface and axis of the cylinder. By this method we are sure that the 

 line preserves invariably the same position, and we may depend upon the dis- 

 tances observed. 



We may ask if the same fault did not exist in the sector with which Bradley 

 made his admirable discoveries of the aberration and nutation. The answer 

 would be the same. For the sector of Bradley, the workmanship of Graham, 

 was the model upon which that celebrated artist had constructed the sector 

 carried to Lapland. Hence Bradley could not depend upon the absolute dis- 

 tances which he measured. Fortunately the error was nearly constant for each 

 star that he observed. Pie required only the relative distances, and the sector 

 gave them almost as exactly as if there had been no error. This fault, which 

 existed certainly in the sector of Lapland, did not prevent Lemonnier, when 

 he returned to France, to observe, as Bradley had done, all the variations pro- 

 duced by aberration, and to confirm fully the brilliant discovery of the English 

 astronomer. See Degre du Meridien entre Paris ct Amims. (Faris, 1T40,) 



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