Irish Observatories.
In Ireland, there have been two observatories fitted up and finished on a great scale ; the one at Dublin, and the other at Armagh.
The observatory belonging to Trinity College, was begun in 1783. It was founded by Dr. F. drews, provost of that college, who bequeathed a large sum for that purpose. The apparatus consists of a transit of six feet focal length, with a four feet axis, ing an aperture of 4a inches, and provided with three different magnifying powers, extending as far as 600. There is also an entire circle of eight feet diameter and moving on a horizontal axis, for measuring chiefly ridian altitudes. In addition to these, there are an torial instrument, with circles of five feet in diameter, and an achromatic telescope mounted on a polar axis, and carried by heliostatic movement. The clocks were furnished by Mr. Arnold, without any limitation of price.
The situation chosen for the observatory is upon an elevated ground, about four miles north-west from Dub lin. The foundation is a solid rock of limestone, of se. veral miles in extent, and the soil is of the most favour able description, being a calcareous substance, called limestone gravel, which absorbs the rain remarkably fast, and thus contributes to render the atmosphere gene rally dry. The plan of the edifice unites at once ele gance and convenience. In the centre is a magnificent dome, three stories high, with a moveable roof for the equatorial instrument, which is placed on a pillar, six teen feet square, of the most substantial masonry, and surrounded by a circular wall at a foot distance, sup porting thc moveable dome as well as the floors, which in no part touch the pillars, to prevent any motion be ing communicated from the floor or wall to the instru ment. The aperture for observation in the dome is two feet and a half wide.
But the most important erection belonging to this establishment is behind the principal building, and at right angles to it, in order to obtain an uninterrupted view of the horizon hoth towards the north and south.
This is the meridian and transit room, which contains both the transit instrument and circle. The dimensions of this room arc 37 feet long, 23 feet broad, and 21 high. For the purpose of supporting both instruments on a firm basis, five pillars of Portland stone have been erect ed, and thc floor is so constructed as to let all the pil lars rise through it totally detached. The clocks are also attached to pillars of the greatest steadiness._ They vvere made by Arnold, who exerted all his skill in their construction, and finished them in a masterly style. The pallets are of ruby, and all the last holes of the move ments arc jewelled. The suspension springs are of gold, with Arnold's fivc-barred pendulum, and cheeks capable of experimental adjustment, so as to make all the oscillations isochronous, whatever may be the ex cursion of the pendulum.
The Rev. Dr. Usher, the first astronomical professor, did not long enjoy the pleasure arising from his labours in astronomy, but died in 1790, before the instruments had been all fitted up. He was succeeded by the Rev. Dr. Brinkley, who had for some time been a pupil of the late Dr. Alaskelyne. Dr. Brinkley studied at Cam bridge, and had distinguished himself there by his pro found analytical investigations. He has since greatly enriched the Philosophical Transactions, and the Trans actions of the Royal Irish ilcadenly, by his valuable ma thematical and astronomical papers.
From observations made with a new eight feet circle, by Ramsden and Berge, important results have been expected, particularly on annual parallax, the aberration of light, and astronomical refraction. Dr. Brinkley has been for some time engaged in a series of observations, with a view to explain the cause of those variations which he has found in the zenith distances of certain stars at different times, which do not seem explicable by any cause yet known. He has found a difference between the zenith distances of a Lyrx, when in oppo sition and conjunction, which may bc explained by a parallax of about two seconds.