Description of the Ramsden Circle (Fig. A.1)
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Appendix A Description of the Ramsden Circle (Fig. A.1 ) A Description of the Ramsden Circle used by Piazzi to discover Ceres, as written by Pearson ( 1829 ). 1 . The fi rst astronomical circular instrument that was made by Ramsden , was that with which the late eminent astronomer, Piazzi of Palermo, took those series of observa- tions, from which the declinations in his much esteemed catalogue were computed and published, fi rst in 1803, and then in an improved state in 1814. Piazzi informs us, that Ramsden twice undertook the construction of this instrument, and as often abandoned it; but at length in January of the year 1788, he entered upon the work in earnest, and fi nished it in August 1789. 2 . The vertical axis of the instrument is composed of various parts, which, being fi rmly united together, constitute a frame that revolves in one piece on two pivots at the extreme ends; the cone, that terminates the lower extremity, tapers from a diameter of 14.2 inches to 5, where it is made fast to the horizontal circle, on which the azimuthal angles are measured, of three feet diameter. This circle has ten tubular radii of a coni- cal form, and is divided into two semicircles, fi gured into 180 degrees, each of which is subdivided into spaces of 6’; it revolves with the frame composing the vertical axis. The base of the inverted cone is fi rmly attached to an oblong plate of metal, which may be called the lower stage, and into which four long vertical tubes are fastened, as so many pillars, to support the upper stage, of the same dimensions as the lower one, namely 25.3 by 16.8 inches. These four pillars are each 6.5 feet long, and 3.5 inches in diam- eter, and, together with the two stages, constitute the frame holding the vertical circle. This circle is fi ve feet in diameter, divided into four successive quadrants, in which each degree is fi gured with an Arabic numeral, and each tenth degree with larger numerals of the Roman characters; and, as the reading microscopes have small fi elds of view, the subdividing strokes, including spaces of 6′, are known by single, double, triple, etc points made contiguous to them on the graduated face. The circumscribing portion of the vertical circle is composed of two fl at rings, standing parallel to and concentric with each other, by means of cross bars that unite them, like so many rounds of a lad- der; which plan gives strength, without adding materially to the weight of the structure. The graduated face is that which is presented to the eye in the perspective fi gure. 3 . The horizontal axis of the vertical circle consists of three pieces, a central cylindrical hollow piece, and two inverted hollow cones, all of brass, compactly fi xed together; at the extreme ends of which two steel cylindrical pivots are made fast, one to the apex of each cone, which bear a portion of the circle’s weight. The eight conical radiating © Springer International Publishing Switzerland 2016 301 C. Cunningham, Discovery of the First Asteroid, Ceres, DOI 10.1007/978-3-319-21777-2 302 Appendix A Description of the Ramsden Circle Fig. A.1 A photo of the author with the Ramsden Circle at Palermo Observatory tubes of brass are fi xed to the central part of the axis at their bases, and at the remote ends to the middle of certain cross bars, connecting the two rings of this circle: the telescope passes through the said central portion, instead of two more radii, and is made fast at both ends between the rings of the double circle, so as to prevent any tendency to fl exure in its tube. The focal distance of the telescope is equal to the diam- eter of the circle; and fi ve direct eye-pieces are supplied, magnifying the linear dimen- sions respectively 50, 75, 100, 130 and 170 times; besides which there is a prismatic eye-piece of the description mentioned in the sixth paragraph of our fi fth section, which performs the offi ce of a refl ecting diagonal eye-piece. This eye-piece has two powers, one of 75, and the other of 130, and is principally used for viewing stars near the zenith. The illumination is effected by transmitting the light of a small lamp through the hollow axis, the inclined refl ector, in the middle of which, is exactly similar to that of a transit-instrument: the light, however, is limited to suit the object viewed by a paralle- lopiped composed of three pieces of glass; of which the middle one is white, and the two extreme ones green; they are contained in a frame that has an adjustable motion by means of pulleys, visible at the remote end of the axis, behind the back pillars. The reason of two green wedges being used is, that the lines in the focus of the eye-piece may not appear double, by passing through two glasses of unequal refracting powers; the second green glass being made to correct the refraction occasioned by the fi rst. The pulleys are acted upon by a long handle terminating with a Hooke’s joint. The whole length of the vertical circle’s axis, including the pivots, is about two feet. Besides the four long pillars already noticed, there are two shorter ones, ascending, at the distance of eleven inches from each other, from the face of the lower stage, up to the middle of Appendix A Description of the Ramsden Circle 303 the inverted cones of the horizontal axis; these hollow pillars are each three feet three inches high, and are braced near their upper ends to their adjoining long pillars; a strong rod passes through each pillar, and supports a small frame, containing a pair of rollers, side by side, one pair of which may be seen under the left hand cone of the axis. Each cone has a circular edge-bar made fast round it, that rests on the Y formed between the pair of rollers, and a pair of adjusting screws acting on the vertical rods, bearing the rollers, force them against the circular rings of the cones, and support any required portion of the circle’s weight, to relieve the pressure on the steel pivots. One of these rods has its adjusting screw under the lower stage, but the other is acted on through a small frame interposed between the two halves of the tube, for some reason that is not obvious. The lower stage is strengthened by several brackets descending from its lower surface to the inferior end of the inverted vertical cone, to which they are made fast. 4 . The two large metallic pillars, that ascend from the two opposite corners of the square, composed of a marble fl oor, are each seven feet high, and four inches in diameter, and are matched by a similar pair, that do not appear, but which ascend from the other two opposite corners of the same square fl oor. These two pillars have arched tops of brass. The arch that connects the fi rst pair of pillars will explain the structure of the second, which is exactly similar, and is placed at right angles to the other. A cross of four straight bars connect the arched portions, that rest on the superior ends of the four pillars, to which they are made fast; and a circular hole, at the place of crossing, receives the tubular pivot at the superior end of the vertical axis, and is fi rmly fi xed to the middle of the oblong opening, that nearly severs the upper stage. This opening allows the telescope to view stars near the zenith without obstruction, the bars, con- necting the two halves of the stage, being thinner than the diameter of the object-glass. 5 . The lower support of the vertical axis consists of three concentric circular plates of iron, laid over one another with attached rollers under the second and third; the upper- most circle bears the conical pivot, on which the axis turns, and the other two have their respective adjustments at right angles to each other; one being moved in the direction of east and west, and the other of north and south: each motion is produced by a horizontal screw, by means of a handle with an universal joint, of which one is seen standing up rather obliquely, to the right of the three circles. The manner of each screw’s action may be easily understood, if we conceive one of them made fast to one of the lower circles and the other made fast to the other, with their tapped ends entering the sides of the uppermost, which may thus be pushed forwards or drawn back in either of the assigned directions. A ring of mahogany, three feet two inches in diameter, and three inches thick is laid over, and made fast to the uppermost circular plate; and forms the basis of a balustrade, having a ring of metal above and another below, connected by twenty cylindrical rods, or small pillars of brass, each thirteen inches long. This balustrade preserves the azimuthal circle from injury, and supplies the means of clamping the vertical axis with a tangent screw of slow motion, which is turned by the handle seen above the balustrade towards the left: it also holds the reading microscope in its proper place, over the divisions of the graduated horizontal circle, that the posi- tion of the telescope, and of the vertical circle, may at any time be indicated by it, when once adjusted so, that zero will show the meridional position.