If 



REPEATING CIRCLE. 





If 



UM reflecting circle. One WM constructed in 1787, and 



connecting the meridian* of I'aris and Greenwich. (See ' Mem. do 



1'AoMlrm. ; a Memoir by Le Qendre, 1797 ; and a memoir by Owrini. 

 The ' Cminnissince dm Tema,' An VI. (177-). contain* the 

 plate and description of a repeating circle which wa made by Lenoir 

 lor th astronomer La Lande. When UM French government under- 

 took UM measurement of an are of the meridian from Dunkirk to 

 Barcelona, the cotnmiatioo to whom this operation wai entrusted 

 resolved to employ the repeating circle. 



Thi. U one of the most complicated a* well as ingenious of existing 

 imtruincnU, and obtained an immense repuUtion. from being the only 

 instrument employed in the gcodesical and astronomical observations 

 of the great meaniremcnt of an arc of the meridian, on which the 

 French have founded their modern system of measures, weighU, and 

 money. Since that tune the oonatruoUon has been altered by different 

 artists, but not alwayi with advantage. 



Fig. 1. BonU'i Repeating Circle. 



la thin figure the general form of the instrument U shown Menblj 

 well, but eome of the essential motions are at the back of the circle, 

 and theee are drawn on a larger Male in a second diagram. The whole 

 circle tunu round on the vertical column, which hag an inner axis of 

 steel, with good fittings at the top and bottom. It is usual and pro(>er 

 to make these fittings with great care, but it U not An essential con- 

 dition to accuracy in the performance of the instrument. 



The top of the column finishes in a square bar, to which the upper 

 works and circle are firmly screwed. We shall first descrilni the 

 motion* which are required for astronomical purposes, and point out 

 the rest when the geodesical properties of this instrument are con- 

 sidered. The azimuthal circle is scarcely to be considered a part of 

 the instrument. 



The front telescope, its verniers, and clamp, are seen distinctly in 

 front (/'!. 1.) This moves very freely on a spindle within the nxU 

 of the circle. There is a level behind, a projecting end uf which i- 

 seen in the figure. Thin and its accompanying back telescope are 

 one a little above and the other a little below the axis of the circle, 

 and revolve on a collar which works on the outaide of that axis. These 

 can t<c fixed in any |>oition by a clamp (also seen hi the figure) which 

 embracee the back edge of the circle : the back telescope is for mea- 

 suring terrestrial angles. Finally, the axis of the circle itself passes 

 through a fitting, on which it alsi> turn* freely, carrying telescopes, 

 level, IK., without altering their position in respect to itself. Tin-re 

 in a clamp to restrain this motion and fix the circle, the head of which 

 is seen at A, .ay. 2, and a tangent screw for slow motion at B. This in 

 the weakest and most inconvenient part of the instrument, fr the 



\ ,:-..,,,- , ! 



assistant h.ivin- previously brought ' 

 ing points of its scale. The level is 

 settle if wantal. We suppose its i 



1 imp holding so near the uxi > has little power, and tli.ro is 



scarcely' room for getting at UM screw-head, while the slow i 

 clamp Is out of the way uf the observer when he requires it for biseo 

 The large weight Miiiid in a counterpoise, and the small level 

 ; for setting UM circle vertical. There is a clamp at < . 

 bites on the semicircle to make Uiis adjustment and prevent |iU 

 being deranged. We will describe the process of observing with the 

 instrument when the object is a star at or near its meridian alt 



Supposing everything to be adjusted, that is, the axis and circle 

 both vertical, the observer bisects the star with the telescope, he or an 



tin- level nearly to the correspond- 

 is now read off, giving it time to 

 suppose its graduation to be in seconds, and 

 reckoned outwards from the centre of the scale. The verniers are 

 read off, the instrument turned half-round on the vertical axis, the 

 telescope clamp released, and the star again bisected by the telescope, 

 using its peculiar clamp and tangent screw ; and finally, the I- 

 again read off. This operation is precisely the same in all circles 

 having an azimuthal motion, ami it is clear that if the verniers were 

 again read off, the difference between the first and second readings 

 would be (after it is corrected for the indication of the level) twice 

 the r.enilh distance of the star. Let the circle be now reversed, the 

 level clamp and circle-axis clamp be released, and the whole circle 

 moved in its plane till the telescope points to the star, and let the star 

 be bisected again by using the axis clamp and its tangent screw only. 

 The level must be brought back to be horizontal while this is doiiiu, 

 and be actually dam/ml before the final bisection of the star U made. 

 nceive that this must be done at twice, even by two observers ; 

 ami it may be done at twice by one, though in a longer time. If the 

 reader has fully understood the process, he will see that the instru- 

 ment is precisely as at the commencment, except that the telescope 

 and its verniers have travelled over the circle, an arc equal to twice 

 the zenith distance of the star. A repetition of the operation ill 

 carry the telescope verniers over four times the distance, and l>y i"ii- 

 tintiing the process the final arc read off may be made any number of 

 times twice the zenith distance of the star. If the series stops after 

 ten such processes, the arc travelled over is twenty times the mmplu 

 zenith distance. Let the verniers be now read off, then subtracting 

 the first reading from the lost, and dividing by twenty, the result will 



Filt. !. H.'i-kof 1 ; iiclo. 



be the zenith dUtanco of the .-tar. .mil tho errors of division or of 

 readme off will also be divided l>y twenty. If the numl. of 

 tions which can be token at one culmination are not thou 

 to destroy these errors, the series maybe prolonged on the following 

 and subsequent nights, starting from the preceding reading without 

 di.it u rbing the verniers, until any number whatever are t-ii, 

 essential condition being that, in reversing, the level in mxli - 

 both in its attachment to the circle and in ita own /em ; .-mil that in 

 bringing the telescope on tho star by turning tho whole on its hori- 

 zontal axis, the connection between tho telescope and circle is perma- 

 nent. The hour, minute, and second at which each bisection is mad e 

 ore to be noted. 



\V. have said that it is Hotter to read off the level than to attempt 

 to bring the ends of the bubble exactly to the same division, \vhi< li : . 

 really an impossible eondit.inn with ar, and scarcely | i 



able even with two. The differences should bo as small an : 

 made with moderate care and in a moderate time ; but a well-ground 



