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THE MERIDIAN CIRCLE.

the stone. Near the western end of the foundation cross arise four other pillars, for the purpose of supporting the frame of the vertical meridian circle. Beneath, from north to south, is laid another massive block of Portland stone, so placed as not to touch the pillars or floor: this is to support the vertical axis. The "clock pillar" is another of these solid supports for that requisite instrument: the clock was made by the late Mr. Arnold especially for this place.

The meridian aperture for the transit instrument and circle, is five feet wide from the horizon to the zenith, and the air is admitted, that the temperature within and without may be equalized; but there is also a thin canvass covering to this opening, which is drawn over, except a space of two feet in the middle. These openings have also shutters, which are only closed in very damp or wet weather.

The south wing is assigned for occasional observations, such as eclipses, occultations, &c.; for comets are always most conveniently observed by the equatorial instrument, which, here, has the entire command of the horizon.

The Meridian Circle.-This circle, which is suspended between the solid pillars already described, is eight feet in diameter; it possesses immense optical power, with wonderful accuracy, both qualities, no doubt, arising from the very great precautions taken in the construction of the instrument, and of the machinery contrived for its suspension. In both of these operations the most consummate skill and discernment have been displayed.

To our readers who may not have seen this circle, or read any description of it, some account of it may be interesting. It is entirely metal, and was commenced by Mr. Ramsden, of London. The view we give, and the following sketch may be interesting:

This splendid instrument is by far the largest of its class that ever has been completed; it is a broad circle, composed solely of brass; it is supported in a frame, which turns on a vertical axis: this axis is a double cone, each portion being four feet in length,

THE MERIDIAN CIRCLE.

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and the pressure of the circle upon it is completely relieved by a very ingenious application of a lever assisted by friction wheels. The circle is divided into intervals of five minutes, which are divided by micrometic microscopes into seconds and parts of seconds. There are also three microscopes attached to it: one of these is opposite to the lower part of the circle, a second opposite the right, and a third opposite the left extremity of the horizontal diameter. By these microscopes the minute subdivisions of the circle, which are indistinct to the naked eye, are marked with the greatest accuracy. From the vast size of the instrument, and the great interval between the upper and lower parts, the temperatures must occasionally differ; and from this cause the relative positions of the points of suspension of the plumb-line (ten feet long) which adjusts the vertical axis and the point below, over which it passes, would experience some change; but to obviate an accident that must be so fatal to the accuracy of the observations, the point of suspension and the point below are on similar compound bars of brass and steel; and hence the distance of the plumb-line from the vertical axis always remains the same; a fact which has been repeatedly and satisfactorily proved, and the remarkable ease and steadiness with which this great circle and its frame turn upon their respective axes by the slightest touch of the fingers, is not only a great advantage in working the instrument, but is also another proof of the beautiful adjustment and balancing of all its members.

It is, however, a subject of much regret to all men of science, that the uncommon slowness of the opticians (already noticed) in completing these instruments should have delayed so long the progress of practical astronomy in Ireland. In affairs of this kind, when twenty years are lost, they cannot easily be regained; however, they now seem to make quick atonement for their long delay, although that delay alone has prevented this observatory from affording the assistance it might have contributed in bringing to their present state, approximating to perfection, the astro

356

PARALLAXES OF SOME FIXED STARS.

nomical tables; almost all which improvements are the results of the observations made during the last eighty years at Greenwich. But Dr. Brinkley was professor from 1792 until 1808, before he had obtained the proper instruments to commence his observations. This was entirely owing to the dilatoriness of the optician, Mr. Ramsden. So that, in fact, Dr. Brinkley may be said to have been curtailed of one half of his professional life, as to the higher objects of astronomical science; for during nearly the first seventeen years after his being elected, he had little to do except in lecturing and examining the astronomical students in the Philosophy school of the University; and then at the end of the next seventeen years he was installed Bishop of Cloyne. However, the moment that Berg, who succeeded Ramsden, had placed the circle in a true state of adjustment, the astronomer commenced his observations, and soon discovered a very important circumstance relative to certain of the fixed stars, which had been suspected to exist, but had not been proved; this was the annual visible parallaxes of the following stars: a Lyra, a Aquila, a Arcturus, and a Cygni. The parallax of the first he set down at =1".0; of the second, 2.7; of the third, 1".10; of the fourth, 1.0. Shewing in the first place, that Aquila, though less brilliant than Lyra, is by one half the distance nearer to us than that star, and that Arcturus is only half the distance of the two others. These results have been obtained from a mean of several hundred observations made at various times, in every season of the year. Should these parallaxes be found correct, it will open a vista into space, almost too vast for the human mind to contemplate,—a distance, compared with which, our solar system would be a mere point in the universe. The ocean of space now open to view being, as Dr. Brinkley assured the author, equal to 200 billions of millions of miles. The details of these operations are to be found at large in

In the year 1799, Dr. Brinkley published a treatise on astronomy, for the use of the students. This work experience has proved to be well calculated for the instruction of that class of pupils.