clamp-screw and slow motion is seen at c, for the In order to fix the transit instrument exactly in th lower circle. meridian, the observatory clock should be previously In using this instrument, the axis of the telescope regulated to sidereal time, by means of corresponding is adjusted to a horizontal line, and the plane of the or equal altitudes of the sun or a star, taken before quadrant to a vertical line, by the means already and after they pass the meridian, by small quadrants mentioned. The screw of the clamp, l, is then loos-or circles, or by a good sextant. The axis of the ened, and the telescope directed to the star whose transit is then to be placed horizontal, by means of a altitude is required. The clamp-screw being fixed, spirit level which accompanies the instrument, and the observer looks through the telescope, and with the greatest care must be taken that the axis of the nut of the screw h, he brings the telescope into a vision describes in the heavens a great circle of the position where the star is bisected by the intersection sphere. of the wires in the field of the telescope. The divisions are then to be read off upon the vernier, and the altitude of the star will be obtained. By means of the horizontal circle, d, b, all angles in the plane of the horizon may be accurately adjusted. pivots of which rest on angular bearings called Y's, The instruments we have now been describing, though of considerable value to the practical astronomer, are unfitted for some of the most important observations he will be called upon to make. On this account we proceed to describe a very important piece of apparatus, combining in itself a variety of instruments, and one that is well fitted for a private observatory. When a simple telescope mounted on a stand, is elevated to view any heavenly body at the moment of its meridian passage, the observed body appears to pass horizontally across the field of view, but in any other situation to the east or west of the meridian, the apparent passage of the body through the field of view is in an oblique direction, and the more so the greater the declination towards the visible pole, and also the greater the distance from the meridian line. Hence, the motion of the simple telescope, which moves in a circle parallel to the horizon, when turned round on its vertical axis, will never coincide with the motion of a heavenly body that moves either in the equator, or in a circle parallel to it; unless indeed the observer could stand at one of the poles, in which case the equator would become the horizon, and circles of altitude would be also circles of declination. But no observer can be so circumstanced. He may, however, incline the axis of motion of his telescope, so as to be placed in the meridian, and exactly parallel to the earth's axis, and then when the field of view is directed to take in any heavenly body, the motion of the telescope round such axis, supposing it to be fixed, will attend the said body during the remainder of its path above the horizon. Accordingly we find Christopher Scheiner using a telescope mounted on a polar axis, in 1620, which was soon after the time that Galileo invented the simple dioptric telescope; and though Muller contrived his torquet, a kind of portable equatorial, in 1544, yet the idea of Scheiner's polar axis most likely suggested to the modern instrument-makers the best principle on which an equatorial instrument, as well as the equatorial sector, ought to be constructed. By the use of Scheiner's contrivance, an observer could follow a star or other heavenly body through its diurnal arc, but it had no appendages to ascertain the place of the observed body, as to right ascension, declination, distance from the meridian, &c.; neither could the telescope be directed to a body invisible to the naked eye. Mr. Henry Hindley, an eminent clock-maker at York, was probably the first man who contrived and attached the different adjustable circles as companions to the telescope. Mr. Smeaton, in his paper on the Graduation of Astronomical Instruments, read before the Royal Society, November 17, 1785, states that Hindley contrived an instrument of the equatorial kind so early as 1741. This instrument had the equatorial plate, quadrant of latitude, and declination semi-circle indented at the circumference, and moved the double vernier bar is in this case fast to the in by worm-screws, containing fifteen threads each, all ferior end of the tube. in action together; which screws at the same time measured as micrometer screws, the angular motions. The telescope was of the refracting kind, and inverted the object viewed. "It staid with me," says Mr. Smeaton, "two years, in which time I showed it to all my mechanical and philosophical friends, amongst whom was Mr. Short, who afterwards published, in the Philosophical Transactions, an account of a portable observatory, but without claiming any particular merit for the contrivance." Mr. Short placed his reflecting telescope over a system of graduated circles, and he has generally been considered as the first contriver of the equatorial instrument, though it appears evident from Mr. Smeaton's account that Hindley's instrument was somewhat prior to his. We come now to Mr. Troughton, who constructed an equatorial instrument sufficiently large to be placed in an observatory for the purpose of making useful observations. This ingenious artist contrived many modifications of the instrument, but the portable equatorial instrument, invented by Mr. Fayrer, will require a more particular description, as its price will admit of its general use in private observatories. This instrument differs from its predecessors chiefly in these respects; it has no azimuth circle fixed in a permanently horizontal position; it has its equatorial circle divided into degrees and also into time, with two verniers reading at opposite points, one vernier reads to the accuracy of 30" of space, and the other to 2" of time; this circle has a long axis passing through a tube, so that it may be placed in a horizontal direction by its axis being placed firmly vertical, in which situation it becomes an azimuth circle; the telescope is fixed on the end of the axis of the declination circle. This position allows any elevation of the telescope that may be required, and keeps the observer's eye at a distance from the other parts of the instrument, but injures the uniformity of the appearance of the instrument as a whole; the levels are applied, one on the common axis of the telescope and declination circle, and the other at right angles thereto, as a chord to the declination circle; and lastly, the declination circle is complete, with opposite verniers reading to about twenty or thirty seconds, according to the dimensions of the graduated circles. The posterior pivot of the horizontal axis, borne by the triangular bars, projects, and has firmly attached to it the quadrantal piece, e, partly hidden from sight in our view of the instrument; this quadrantal piece is graduated, and reads by a vernier as minutely as the other circles; it serves to set the equatorial axis parallel to the earth's axis, in which situation it may be clamped fast, as well as in a vertical, or indeed in any other position, by a clampingscrew out of sight in the present position, applying to the quadrantal arch; the declination circle is shewn at f, with its horizontal axis resting on a pair of vertical supports, carried by the upper end of the polar axis, that passes through the tube c, so that, when the telescope, attached to the axis of the declination circle, has any horizontal or rather oblique motion, the other circle moves with it, and indicates the distance moved along the equator, or in a circle parallel to it. The double vernier bar is on the posterior place of the declination circle, and may therefore be viewed The adjustment screws are shown at a. The tri- after an observation without danger of altering the angular frames, b, are fixed firmly to the circular position of the telescope. The level on the declinastand, and support the pivots of the horizontal axis, tion axis may be conveniently adjusted by turning round which the tube c moves. At the summit of the circle half round, and by making the bubble keep the triangular frame b, is an adjustable or sliding the middle of the tube in both situations, which may part, like that of a transit instrument, by which the be done, partly by the screws a, and partly by the telescope may be adjusted in the meridian, when pre-screws of rectification of the level itself; and zero of viously placed nearly so by the feet. The graduated circle, which may be either an equatorial or azimuth circle, accordingly as it is inclined or vertical, has a steel axis nicely fitted to revolve in the tube in any required position with respect to the horizon; but From such a construction it is easy to perceive that this little instrument possesses the advantage of having its telescope reversed as well as its circles, with respect to the opposite verniers, which property greatly ensures the accuracy of the adjustments. After these previous remarks, it will suffice to give a brief detail of the parts of this instrument, as represented in the accompanying figure. the declination circle may be put to zero on the verniers when the circle is truly placed in an horizontal position, in which situation also zero of the quadrant e, must coincide with the zero on its vernier. An instrument thus constructed, will possess al the va rious powers ascribed to a portable equatorial, combined with the other instruments. reflector of silver for the illumination of the dividing marks of the azimuth circle: the field of view of the We have now to furnish an account of the most compound microscope contains but a very small space important astronomical instrument which the inge- of the image of the divided limb; it was therefore nuity and mechanical skill of this country has pro- found necessary, not only to mark every degree with duced; we allude to the great circle of Mr. Ramsden, ten successive Arabic numerals, and also each tenth which has been very fully described by Piazzi. It is space, with larger numerals of the Roman character, represented in our Plate, ASTRONOMY, fig. 1. The but also to insert points for discriminating the ten vertical axis of this instrument is composed of various subdividing lines, which are counted 0, 1, 2, 3, &c., parts, which revolve together, and which may be con- the distance between each of which, we have already sidered, when firmly united, as one piece; at the lower said is 6', therefore, the corresponding values are 0', end is a cone, I, inverted, the smallest diameter of 6', 12, 18', 24', &c. up to a degree, as read without which is five inches, where it is attached to the azi- the aid of the micrometer. muth or horizontal circle with a series of conical radii, and the greatest diameter is 14.2, where it is fixed to the oblong stage of brass, A; which stage is further strengthened by suspension pieces, at the four corThe azimuth circle is three feet in diameter, divided into 180° twice over, and each of the degrees again into ten subdivisions of 6′ each. The extreme inferior end of the axis, below the azimuth circle, is a small cone of hard steel. On the stage A, are fastened four strong brass pillars, indicated by the letters C C C C, and placed near the corners of the stage. Above the superior ends of these four pillars, is another stage, B, of similar dimensions, in the centre of which is a tube, which constitutes the upper pivot of the axis; at each side of the central tube of this upper stage, is an opening which nearly divides the stage in two, except at the middle and two extreme edges, which edges are made firm by lateral connecting pieces; the use of the open parts of the upper stage is to admit the object end of the telescope, to view stars near the zenith. ners. The compound microscope, N, has the mechanism of the micrometer, in the point where the focus of the eye-glass, or perhaps we should rather say, where the united foci of the glasses of the compound eyepiece meet the image of the subdivisions of the limb, as formed in the tube by the object-lens ; this mechanism is rather complex, and cannot be very clearly apprehended, perhaps, by a mere verbal description; it consists of two parallel horizontal plates of metal, having each an oblong hole along its middle, the upper one of brass and the lower of steel; the brass one is divided into ten spaces of each 1', counted each way from zero, which is a point in the middle, and is moveable separately by the horizontal screw on the left hand; the steel plate carries a cross hair or wire, and is adjustable to the right or left by a screw of 70 threads per inch, which has a nut, as a head, divided into 60 equal parts, one of which parts corresponds to a second of a degree; this divided head is placed at the right hand of the microscope, so that one of the two screws cannot be mistaken for the other, and both may be held at the same time, and turned by the separate hands of an observer, if necessary. To prevent a loss of motion in the screw of divided head, or micrometer-screw, a spring of contrary pressure is applied in constant action, which makes the cross wire move backwards or forwards, without the loss of even a second, as counted on the divided head. The two large pillars, each 7 feet high, and 4 inches diameter, ascending from marble bases on the floor of the observatory, and terminating with a large arch, which connects their superior ends, constitute a part of the superior support of the vertical axis; two similar pillars, omitted in the engraving, also rest on the marble base at right angles to the above. At the top of the arches, a cross or piece of four straight bars, is screwed to the four upper portions of the arches, and a hole in the centre of this uppermost cross piece receives the tubular pivot of the vertical axis. The lower support of the vertical axis consists of three concentric circles of iron, laid one over another on friction rollers; the uppermost of which bears the inferior pivot of the axis, and the other two have each an adjustable motion, one from east to west, and the other from north to south. These are moved by handles attached to fine screws, which screws acting as pressing points, move the large iron circles in their respective directions, when the axis is to be placed exactly perpendicular to a horizontal line drawn in any azimuth. M is a mahogany circle attached to the uppermost iron circle; on this circle of wood is placed a balustrade of metal, R R, composed of a superior and inferior large ring, each being three feet in diameter, connected by twenty cylindrical pillars, each of one inch diameter, and 13 inches high; this balustrade defends the azimuth circle, and serves to give either a slow or quick motion from it to the axis of the instrument, by means of the clamping mechan-are composed of eight metallic cones and the teleism, connected with an universal joint, of which the handle Q only is seen in the figure. The microscopic micrometer, N, which reads off the graduations of the azimuth circle, is also carried between two of the pillars of this balustrade, together with the subjoined The microscopes have two adjustments, one for the object-lens to make the image fall distinctly on the micrometer's thread and scale, and another for the eye-piece to render this image clear to the eye; also the micrometer has two adjustments, one to adjust zero of the scale, under the eye-piece, to zero of the image of the divided limb: these two latter adjustments are effected by the different fixing screws, which are not seen in the figure. The circumscribing boundary of the circle, corresponding to the felley of a wheel, is formed of two separate rings, united in various equidistant points by parallel cylindrical pieces, so that the appearance of the compound piece is that of a circular ladder; a form which gives strength without any great addition to the weight. On the plane of one of these rings is firmly fixed a third circle, which contains the lines of graduation, which are but faintly seen in the figure. The central piece, or nave of the wheel, into which the spokes, or radii, are fast, is a segment of a cylinder of cast brass, nicely perforated in the middle, and the spokes scope, which passes through the nave and forms two more. The horizontal axis of this large circle, or wheel, as we have described it, is formed of a double cone, which is hollow throughout, and has pivots of hard steel at the extreme ends; it has four supports, |