of f/ie late Mr. Rumsdtn. 261 



the dlficrencc arose from a mural quadrant of Bird, Avhcr« 

 tlu; arc of DO degrees contained several seconds too much, and 

 ^\ iiich had not been verified bv so exact a method as his. 



fiut the vjLiadrant is not tlie instrument which Mr. Rams- 

 dea values most. It is the whole circle; and he i)as proved 

 that to attain to the utmost degree of precision of which 

 observation is susceptible, we must renounce the quadrant 

 entirely. His principal reasons arc, 1st, llie least variation 

 in the centre is perceived by the two points diametrically op- 

 posite. 2d, As the circle is turned the plane is always ri- 

 gorously exact; which cannot possil)lv be the case in the 

 ijuadrant. 3d, Two measurements can alwavs be had of 

 the same arc; which serves for verifying the accuracy of the 

 observation. 4th, The first point of the divisi(ni can be 

 verified every day witli the greatest ease, oth, The dilata- 

 tion of the metal is uniforut, and can produce no error. 6th, 

 'I'his instrument is a meridian telescope as well as a mural. 

 7th, It becomes a moveable azimuth circle by adding a 

 horizontal circle below the axis, and then gives the refrac- 

 tions independcntlv of the measure of time. You there- 

 fore approved the resolution I had formed of confining m\'- 

 self to this instrument, and of not (juitting London till I 

 could carry with me a circle of five feet, which Mr. Rams- 

 den was constructing for the observatory^ of Palermo : as 

 soon as mine is finished he promises to put in hand that 

 destined for Paris. Jic tlien hopes to finish that for Dublin, 

 which is twelve feet, and wliich vou saw in an advanced 

 state; but a circle of 7 or 8 feet is sufficient to give preci- 

 sion within half a second, as in the zenith sector, which is 

 employed for tlie most rigorous observations in regard to 

 tlie figure of the earth. 



You remarked with the greatest satisfaction the ingenious 

 manner in which the axis is supported, that it may have no 

 friction on the pivots, and especially Mr. Ramsdcn's new 

 invention for rendering the axis perfectly horizontal, by 

 means ot a thread and plunnnet, which is however with- 

 out tlie njachine ; and you had the pleasure of seemg that 

 inventive genius exercise his talents on this new problem, 

 and solve it in the completcst numner. As his talents have 

 been exercised in a wide field, he has collected in his shops 

 workmen in all professions wliich relate to the construction 

 of mathematical and philosophical instruments, that he 

 niiglit have every thing made under his own inspection. 

 '1 he same w«)rkman is alwavs confined to the same brajich, 

 aiul by these means acipiires the utmost degree of correct- 

 ness. But notwithstanding this jH'rfcclion, whiuh ought to 

 U ;{ enable 



