Transit Instrument by Jones

Transit Instrument by Thomas Jones. 1827. 3-inch aperture, 63 inches focal length. This instrument was described by T.R. Robinson in the Armagh Observations for 1828 and 1829. Power of instrument: 120 diameters, changed in 1840 to 195 diameters.

"The transit was supported by piers of Armagh marble, 6' high, 24" by 18" at their base, 12" by 18" at top, secured by cement and bronze dowels to a block of the same marble, 6' by 5' and 10" thick.

A small circle made by Mr. Gardner and divided by Mr. Grubb has been attached to the axis of the Transit. The tube and the cones of the axis are connected by tension-screws, as described by Sir James South (Phil. Trans. Roy. Soc., 1826, Pt.III, p. 423); the braces shown in his figure are not applied. The axis is 30" long, and the central ball 9" in diameter. The pivots are 1" diameter, of bell-metal. The instrument has, at the eye-end, Troughton's altitude circles, but a circle 8.5" diameter, was later attached by Grubb of Dublin, to the perforated end of the axis, in 1844. The Y's are carried by strong semicircular disks of brass 13" diameter and 1" thick, let into the piers flush with their surface. The adjustment for level is made at the western Y by check-nuts on a screw; that for azimuth at the other Y. The surfaces of the Y's are inclined at 60 degrees instead of 90 degrees, to reduce the effects of friction, and are faced with Brazil pebble. The counterpoise levers were originally similar to those which Troughton had applied to the Greenwich transit, however, they were seen to be unsuitable. The rollers were removed from the annular levers and replaced by gibbets, from which were suspended Y's, faced with agate, to take the projecting parts of the pivots". In October 1832, Thomas Jones came to Armagh to re-divide the circle, and Robinson availed himself of the opportunity to have the pivots reground. The pivots were again examined on 25 May 1839 on the occasion of a visit by Mr Dent. "

The Transit Instrument and Jones Mural Circle were used by Robinson to compile the publication: "Places of 5345 Stars observed from 1828 to 1854 at the Armagh Observatory". The main purpose of this catalogue was to re-determine the positions of the stars observed by James Bradley in the middle of the 18th century, to which a number of other stars, chiefly from Lalande, had been added. Several special investigations were also entered into, such as the determination of longitude of the Observatory by chronometers, rocket signals and other methods (see Mem. Roy. astr. Soc., 4, 293-304; Trans. Roy. Irish Acad., 19, 110-146).

Last Revised: 2009 November 5th