figure 87.
figure 87.
Every circle being supposed to be divided into 360 equal parts, or degrees,—it is evident, that 90 degrees, or the fourth part of a circle, will be sufficient to measure all angles, between the horizon of any place and the line perpendicular to it which goes up to the zenith. Thus, in fig. 87, the line CB represents the plane of the horizon. ACBH, the quadrant, AC the perpendicular to the horizon, and A the zenith point. If the lines BC and CA represent a pair of compasses with the legs standing perpendicular to each other, and the curved lines AB, DE and FG, the quarter of as many circles of different sizes—it is evident that although each of these differs from the others in size, yet that each contains the same portion of a circle, namely a quadrant or fourth part; and thus it would be from the smallest to the largest quadrant that could be formed,—theywould all contain exactly 90 degrees each. By the application of this principle the comparative measure of angles may be extended to an indefinite distance. By means of an instrument constructed in the form of a quadrant of a circle, with its curved edge divided into 90 equal parts, the altitude of any object in the heavens can at any time be determined.
There are various constructions of this instrument, some of them extremely simple, and others considerably complex and expensive, according to the degree of accuracy which the observations require. The following is a description of thePillar Quadrant, as it was made by Mr. Bird, for the observatory of Greenwich, and several continental observatories.
This instrument consists of a quadrant E E H G L (fig. 88.) mounted on a pillar B, which is supported by a tripod AA, resting on three foot screws. The quadrant, the pillar, and the horizontal circle all revolve round a vertical axis. A telescope H is placed on the horizontal radius, and is directed to a meridian mark previously made on some distant object for placing the plane of the instrument in the meridian, and also for setting the zero, or beginning of the scale truly horizontal. This is sometimes done by a level instead of a telescope, and sometimes by a plumb-line G, suspended from near the centre, and brought to bisect a fine dot made on the limb, where a microscope is placed to examine the bisection. The weight or plummet at the end of the plumb-line is suspended in the cistern of waterb, which keeps it from being agitated by the air. A similar dot is made for the upper end of the plumb-line upon a piece of brass, adjustable by a screwd, in order that the line may be exactlyat right angles to the telescope, when it is placed ato. The quadrant is screwed by the centre of its frame, against a piece of brassewith three screws, and this piece is screwed to the top of the pillar B with other three screws. By means of the first three screws, the plane of the quadrant can be placed exactly parallel to the vertical axis, and by the other screws the telescope H can be placed exactly perpendicular to it. The nut of the delicate screw L is attached to the end of the telescope F, by a universal joint. The collar for the other end is jointed in the same manner to aclamp which can be fastened to any part of the limb. A similar clamp-screw and slow motion is seen atnfor the lower circle, which is intended to hold the circle fast, and adjust its motion. The divisions of the lower, or horizontal circle, are read by verniers, or noniuses, fixed to the arms of the tripod atlandm, and, in some cases three are used to obtain greater accuracy.
figure 88.
figure 88.
In using this quadrant, the axis of the telescope H is adjusted to a horizontal line, and the plane of the quadrant to a vertical line, by the means already stated. The screw of the champ L is then loosened, and the telescope directed to the star, or other object, whose altitude is required. The clamp screw being fixed, the observer looks through the telescope, and with the nut of the screw L he brings the telescope into a position where the star is bisected by the intersection 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, all angles in the plane of the horizon may be accurately measured—such as the amplitudes and azimuths of the celestial bodies.
Quadrants of a more simple construction than the above, may be occasionally used, such as Gunter’s, Cole’s, Sutton’s and others; but none of these are furnished with telescopes, or telescopic sights, and therefore an altitude cannot be obtained by them with the same degree of accuracy as with that which has been now described.
By means of the Quadrant, not only the altitudes of the heavenly bodies may be determined, but also the distances of objects on the earth by observations made at two stations—the altitude of fireballs and other meteors in the atmosphere—theheight of a cloud, by observation on its altitude and velocity—and numerous other problems, the solution of which depends upon angular measurements. AMural Quadrantis the name given to this instrument when it is fixed upon a wall of stone, and in the plane of the meridian, such as the quadrant which was erected by Flamstead in the Observatory at Greenwich. Although the quadrant was formerly much used in astronomical observations, yet it may be proper to state, that its use has now been almost completely superseded by the recent introduction ofAstronomical Circles, of which we shall now give the reader a very short description, chiefly taken from Troughton’s account of the instrument he constructed, as found in Sir D. Brewster’s Supplement to Ferguson’s Astronomy.
figure 89.
figure 89.
An astronomical circle is acomplete circlesubstituted in place of the quadrant, and differs from it only in the superior accuracy with which it enables the astronomer to make his observations. The large vertical or declination circle CC (fig. 89.) is composed of two complete circles strengthened by an edge bar on their inside, and firmly united at their extreme borders by a number of short braces or bars which stand perpendicular between them, and which keep them at such a distance as to admit the achromatic telescope TT. This double circle is supported by 16 conical bars, firmly united along with the telescope, to a horizontal axis. The exterior limb of each circle is divided into degrees and parts of a degree, and these divisions are divided into seconds by meansof the micrometer microscopesmm, which read off the angle on opposite sides of each circle. The cross wires in each microscope may be moved over the limb till they coincide with the nearest division of the limb, by means of the micrometer screwscc, and the space moved through is ascertained by the divisions on the graduated head abovec, assisted by a scale within the microscope. The microscopes are supported by two arms proceeding from a small circle concentric with the horizontal axis, and fixed to the vertical columns. This circle is the centre upon which they can turn round nearly a quadrant for the purpose of employing a new portion of the divisions of the circle, when it is reckoned prudent to repeat any delicate observations upon any part of the limb. Athis represented a level for placing the axis in a true horizontal line, and atkis fixed another level parallel to the telescope, for bringing the zero of the divisions to a horizontal position. The horizontal axis to which the vertical circle and the telescope are fixed, is equal in length to the distance between the vertical pillars, and its pivots are supported by semicircular bearings, placed at the top of each pillar. These two vertical pillars are firmly united at their bases to a cross barf. To this cross bar is also fixed a vertical axis about three feet long, the lower end of which terminating in an obtuse point, rests in a brass conical socket firmly fastened at the bottom of the hollow in the stone pedestal D, which receives the vertical axis. This socket supports the whole weight of the moveable part of the instrument. The upper part of the vertical axis is supported by two pieces of brass, one of which is seen ate, screwed to the ringi, and containing a right angle, or Y. At each side of the ring, opposite to the points of contact, is placed a tube containing a heliacal spring, which, by a constant pressure on the axis, keeps it against its bearings, and permits it to turn, in these four points of contact, with an easy and steady motion. The two bearings are fixed upon two rings capable of a lateral adjustment; the lower one by the screwdto inclinethe axis to the east or west, while the screwbgives the upper oneia motion in the plane of the meridian. By this means the axis may be adjusted to a perpendicular position as exactly as by the usual method of the tripod with feet screws. These rings are attached to the centre pieces, which is firmly connected with the upper surface of the stone by six conical Tubes A, A, A, &c., and brass standards at every angle of the pedestal. Below this frame lies the azimuth circle EE consisting of a circular limb, strengthened by ten hollow cones firmly united with the vertical axis, and consequently turning freely along with it. The azimuth circle EE is divided and read off in the same manner as the vertical circle. The arms of the microscopes BB project from the ringi, and the microscopes themselves are adjustable by screws, to bring them to zero and to the diameter of the circle. A little above the ringiis fixed an arm L which embraces and holds fast the vertical axis with the aid of a champ screw. The arm L is connected at the extremity with one of the arms A, by means of the screwa, so that by turning this screw, a slow motion is communicated to the vertical axis and the azimuth circle.
In order to place the instrument in a true vertical position, a plumb-line, made of fine silver wire, is suspended from a small hook at the top of the vertical tuben, connected by braces with one of the large pillars. The plumb-line passes through an angle in which it rests, and by means of a screw may be brought into the axis of the tube. The plummet at the lower end of the line is immersed in a cistern of watert, in order to check its oscillations, and is supported on a shelf proceeding from one of the pillars. At the lowerend of the tubenare fixed two microscopesoandp, at right angles to one another, and opposite to each is placed a small tube containing a lucid point. The plumb-line is then brought into such a position by the screwsd,b, and by altering the suspension of the plumb-line itself, that the image of the luminous point, like the disk of a planet, is formed on the plumb-line, and accurately bisected by it. The vertical axis is then turned round, and the plumb-line examined in some other position. If it still bisects the luminous point, the instrument is truly vertical; but if it does not, one half of the deviation must be corrected by the screwsdb, and the other half by altering the suspension of the line till the bisection of the circular image is perfect in every position of the instrument.
It is not many years since Circular Repeating instruments came into general use. The principle on which the construction of a repeating circle is founded appears to have been first suggested by Professor Mayer of Gottingen, in 1758; but the first person who applied this principle to measure round the limb of a divided instrument, was Borda, who about the year 1789, caused a repeating circle to be constructed that would measure with equal facility horizontal and vertical angles. Afterwards, Mr. Troughton greatly improved the construction of Borda’s instrument by the introduction of several contrivances which ensure, at the same time, its superior accuracy and convenience in use; and his instruments have been introduced into numerous observatories. Circular instruments, on a large scale, have been placed in the Royal Observatory of Greenwich, and in most of the principal observatories on the continent of Europe. Although it is agreed on all hands thatgreater accuracy may be obtained by a repeating circle, than by any other having the same radius, yet there are some objections to its use which do not apply to the altitude and azimuth circle. The following are the principal objections, as stated in Vol. I., of the ‘Memoirs of the Astronomical Society of London.’ 1. The origin of the repeating circle is due tobad dividing, which ought not to be tolerated in any instrument in the present state of the art. 2. There are three sources of fixed error which cannot be exterminated, as they depend more on the materials than on the workmanship; first, the zero of the level changes with variations of temperature; secondly, the resistance of the centre work to the action of the tangent screws; and thirdly, the imperfection of the screws in producing motion, and in securing permanent positions. 3. The instrument is applied with most advantage to slowly moving or circumpolar stars; but in low altitudes these stars are seen near the horizon, where refraction interferes. 4. Much time and labour are expended, first in making the observations, and again in reducing them. 5. When any one step in a series of observations is bad, the whole time and labour are absolutely lost. 6. When the instrument has a telescope of small power, the observations are charged with errors of vision, which the repeating circle will not cure. 7. This instrument cannot be used as a transit instrument, nor for finding the exact meridian of a place.
A great variety of directions is necessary in order to enable the student of practical astronomy thoroughly to understand and to apply this instrument to practice, which the limited nature of the present work prevents us from detailing.—As this instrument consists of a variety of complicatedpieces of machinery, it is necessarily somewhat expensive. A six inch brass astronomical circle for altitudes, zenith or polar distances, azimuths, with achromatic telescope, &c., is marked in Messrs. W. and S. Jones’ catalogue of astronomical instruments, at £27 6s. A circle 12 inches diameter, from £36 15s. to £68 5s. An 18 inch ditto, of the best construction, £105. The larger astronomical circles for public observatories, from 100 to a 1000 guineas and upwards, according to their size, and the peculiarity of their construction.
A Transit instrument is intended for observing celestial objects as they pass across the meridian. It consists of a telescope fixed at right angles to a horizontal axis—which axis must be so supported that what is calledthe line of collimation, or the line of sight of the telescope, may move in the plane of the meridian. This instrument was first invented by Romer in the year 1689, but has since received great improvements by Troughton, Jones and other modern artists. Transit instruments may be divided into two classes,Portable, andFixed. The portable instrument, when placed truly in the meridian, and well adjusted, may be advantageously used as a stationary instrument in an observatory, if its dimensions be such as to admit of a telescope of 3½ feet focal length; but when the main tube is only from 20 to 30 inches long, with a proportional aperture, it is more suited for a travelling instrument to give the exact time; and, when carried on board a ship in a voyage of discovery, may be taken on shore atany convenient place, for determining the solar time of that place, and for correcting the daily rate of the Chronometer giving the time at the first meridian, so that the longitude of the place of observation may be obtained from the difference of the observed and indicated times, after the proper corrections have been made.
figure 90.
figure 90.
The following is a brief description of one of Mr. Troughton’s Portable Transit Instruments. In fig. 90. PP is an achromatic telescope firmly fixed, by the middle to a double conical and horizontal axis HH, the pivots of which rest onangular bearings called Ys, at the top of the standards B, B, rendered steady by oblique braces DD, fastened to the central part of the circle, AA. In large fixed instruments, the pivots and angular bearings are supported on two massive stone pillars, sunk several feet into the ground, and are sometimes supported by mason-work, to secure perfect stability. The axis HH has two adjustments, one for making it exactly level, and the other for placing the telescope in the meridian. A graduated circle L is fixed to the extremity of the pivot which extends beyond one of the Ys, and the two radii that carry the verniersaa, are fitted to the extremities of the pivot in such a way as to turn round independent of the axis. The double verniers have a small level attached to them, and a third armb, which is connected with the standard B by means of a screws. If the verniers are placed by means of the level, in a true horizontal position, when the axis of the telescope is horizontal, and the armbscrewed by the screwsto the standard B, the verniers will always read off the inclination of the telescope, and will enable the observer to point it to any star, by means of its meridian altitude. The whole instrument rests on three foot screws entered into the circle AA. In the field of view of the telescope, there are several parallel vertical wires, crossed at right angles with a horizontal one, and the telescope is sometimes furnished with a diagonal eye-piece, for observing stars near the zenith. A level likewise generally accompanies the instrument, in order to place it horizontal, by being applied to the pivots of the axis.
In order to fix the transit instrument exactly in the meridian, a good clock regulated to sidereal time is necessary. This regulation may be effectedby taking equal altitudes of the sun or a star before and after they pass the meridian, which may be done by small quadrants, or by a good sextant. The axis H of the instrument is then to be placed horizontal by a spirit level, which accompanies the transit, and the greatest care must be taken that the axis of vision describes in the heavens a great circle of the sphere. To ascertain whether the telescope be in the plane of the meridian, observe by the clock when a circumpolar star seen through the telescope transits both above and below the pole; and if the times of describing the eastern and western parts of its circuit be equal, the telescope is then in the plane of the meridian; otherwise, certain adjustments must be made. When the telescope is at length perfectly adjusted, a land-mark must be fixed upon, at a considerable distance—the greater the better. This mark must be in the horizontal direction of the intersection of the cross wires, and in a place where it can be illuminated, if possible, in the night time, by a lantern hanging near it; which mark being on a fixed object, will serve at all times afterwards for examining the position of the telescope.
Various observations and adjustments are requisite in order to fixing a transit instrument exactly in the plane of the meridian. There is the adjustment of thelevel—the horizontal adjustment of the axis of the telescope—the placing of the parallel lines in the focus of the eye-glass, so as to be truly vertical, and to determine the equatorial value of their intervals—the collimation inazimuth, so that a line passing from the middle vertical line to the optical centre of the object-glass, is at right angles with the axis of the telescope’s motion—the collimation inaltitude, so that the horizontal lineshould cross the parallel vertical lines, not only at right angles, but also in the optical centre of the field of view—with various other particulars; but of which our limited space will not permit us to enter into details. Those who wish to enter into all the minute details in reference to the construction and practical application of this and the other instruments above described, as well as all the other instruments used by the Practical Astronomer, will find ample satisfaction in perusing the Rev. Dr. Pearson’s Introduction to Practical Astronomy, 4to., Vol. II.
A portable Transit instrument, with a cast-iron stand, the axis 12 inches in length, and the achromatic telescope about 20 inches, packed in a case, sells at about 16 guineas: with a brass-framed stand and other additions, at about 20 guineas. Transit instruments of larger dimensions are higher in proportion to their size, &c.
In order to make observations, with convenience and effect, on the heavenly bodies, it is expedient that anobservatory, or place for making the requisite observations, be erected in a proper situation. The following are some of the leading features of a spot adapted for making celestial observations: 1. It should command an extensive visible horizon all around, particularly towards the south and the north. 2. It should be a little elevated above surrounding objects. 3. It should be, if possible, at a considerable distance from manufactories, and other objects which emit much smoke or vapour, and even from chimney-tops where no sensible smoke is emitted, as the heated air from the top of funnels causes undulations in the atmosphere. 4. It should be at a distance from swampy ground or valleys that are liable to be covered with fogs and exhalations. 5. It should not, if possible, be too near public roads, particularly if paved with stones, and frequented by heavy carriages, as in such situations, undulations and tremulous motions may be produced, injurious to the making of accurate observations with graduated instruments. 6. It is expedient that the astronomical observer should have accessto some distant field within a mile of the observatory, on which a meridian mark may be fixed, after his graduated instruments are properly adjusted. The distance at which a meridian mark should be erected will depend in part on the focal length of the telescope generally used for making observations on the Right Ascensions and declinations of the stars. It should be fixed at such a distance that the mark may be distinctly seen without altering the focus of the telescope when adjusted to the sun or stars, which, in most cases, will require to be at least half a mile from the place of observation, and more if it can be obtained.
Observatories may be distinguished into public and private. Aprivateobservatory may be comprehended in a comparatively small building, or in the wing of a building of ordinary dimensions for a family, provided the situation is adapted to it. Most of our densely-peopled towns and cities, which abound in narrow streets and lanes, are generally unfit for good observatories, unless at an elevated position at their extremities. Public observatories, where a great variety of instruments is used, and where different observers are employed, require buildings of larger dimensions, divided into a considerable number of apartments. The observatory ofGreenwichis composed principally of two separate buildings—one of which is the observatory properly so called, where the assistant lives and makes all his observations; the other is the dwelling-house in which the astronomer-royal resides. The former consists of three rooms on the ground-floor, the middle of which is the assistant’s sitting and calculating room, furnished with a small library of such books only as are necessary for his computations, and anaccurate clock made by the celebrated Graham, which once served Dr. Halley as a transit-clock. Immediately over this is the assistant’s bed-room, with an alarum to awake him to make his observations at the proper time. The room on the eastern side of this is called thetransit-room, in which is an 8 feet transit instrument, with an axis of 3 feet, resting on 2 pieces of stone, made by Mr. Bird, but successively improved by Messrs. Dollond, Troughton and others. Here is also a chair to observe with, the back of which lets down to any degree of elevation that convenience may require. On the western side is thequadrant room, with a stone pier in the middle running north and south, having on its eastern face a mural quadrant of 8 feet radius, by which observations are made on the southern quarter of the meridian, through an opening in the roof, of 3 feet wide, produced by means of two sliding shutters. On the western face is another mural quadrant of 8 feet radius, the frame of which is of iron, and the arch of brass, which is occasionally applied to the north quarter of the meridian. In the same room is the famous zenith sector, 12 feet long, with which Dr. Bradley made the observations which led to the discovery of the nutation of the earth’s axis and the aberration of the light of the fixed stars. Here are also Dr. Hooke’s reflecting quadrant and three time-keepers by Harrison. On the south side of this room a small wooden building is erected for the purpose of observing the eclipses of Jupiter’s satellites, occultations of stars by the moon, and other phenomena which require merely the use of a telescope, and the true or mean time. It is furnished with sliding shutters on the roof and sides to view any part of the hemisphere from thePrime Vertical down to the southern horizon. It contains a 40-inch achromatic, with a triple object-glass; and also a 5 feet achromatic by Messrs. John and Peter Dollond—a 2 feet reflecting telescope by Edwards, and a 6 feet reflector by Herschel. Above the dwelling-house is a large octagonal room, which is made the repository for certain old instruments, and for those which are too large to be used in the other apartments. Among many other instruments, it contains an excellent 10 feet achromatic by Dollond, and a 6 feet reflector by Short. Upon a platform, in an open space, is erected the great reflecting telescope constructed by Mr. Ramage of Aberdeen, on the Herschelian principle, which has a speculum of 15 inches diameter, and 25 feet focal length, remarkable for the great accuracy and brilliancy with which it exhibits celestial objects. Various other instruments of a large size, and of modern construction, have of late years been introduced into this observatory, such as the large and splendid transit instrument constructed by Troughton, in 1816—the two largemural circlesby Troughton and Jones—the transit clock, by Mr. Hardy, and several other instruments and apparatus which it would be too tedious to enumerate and describe.
Every observatory, whether public or private, should be furnished with the following instruments. 1. A transit instrument for observing the meridian passage of the sun, planets and stars. 2. A good clock whose accuracy may be depended upon. 3. An achromatic telescope, at least 44 inches focal distance, with powers of from 45 to 180 for viewing planetary and other phenomena—or, a good reflecting telescope at least 3 feet long, and the speculum 5 inches diameter. 4. An equatorial instrument, for viewing the stars andplanets in the day-time, and for finding the Right Ascension and declination of a comet, or any other celestial phenomenon. Where this instrument is possessed, and in cases where no great degree of accuracy is required, the equatorial may be made to serve the general purposes of a transit instrument.
A private observatory might be constructed in any house which has a commanding view of the heavens, provided there is an apartment in it, in which windows may be placed, or openings cut out fronting the north, the south, the east and the west. The author of this work has a small observatory erected on the top of his house, which commands a view of 20 miles towards the east, 30 miles towards the west, and north-west, and about 20 miles towards the south, at an elevation of above 200 feet above the level of the sea, and the banks of the Tay, which are about half a mile distant. The apartment is 12½ feet long by 8½ wide, and 8½ feet between the floor and the roof. It has an opening on the north by which observations can be made on the pole-star; a window on the south by which the meridian-passages of the heavenly bodies may be observed; another opening towards the east, and a fourth opening, consisting of a door, towards the west. There is a pavement of lead on the outside, all around the observatory-room, enclosed by a stone parapet 3½ feet high, the upper part of which is coped with broad flat stones, in certain parts of which groves or indentations are made for receiving the feet of the pedestal of an achromatic telescope, which form a steady support for the telescope in the open air, when the weather is calm and serene, and when observations are intended to be made on any region of the heavens. By placing aninstrument on this parapet, it may be directed to any point of the celestial canopy, except a small portion near the northern horizon, which is partly intercepted by a small hill. In the following ground-plan, fig. 91. AAA, is the parapet surrounding the observatory-room; BBB, a walk around it nearly 3 feet broad, covered with lead. O is the apartment for the observatory, having an opening C to the north, another opening D to the east, E is a window which fronts the south, and F is a door fronting the west, by which an accessis obtained to the open area on the outside. GHI is an area on the outside towards the south, covered with lead, 15 feet long from G to H, and 6½ feet from E to I, from which a commanding view of the southern, eastern and western portions of the heavens may be obtained:eeeeare positions on the top of the parapet where a telescope may be conveniently placed, when observations are intended to be made in the open air. The top of this parapet is elevated about 30 feet from the level of the ground. On the roof of the observatory, about 12 feet above its floor, on the outside is a platform of lead, surrounded by a railing, 6 feet by 5, with a seat, on which observations either on celestial or terrestrial objects may occasionally be made. K is a door or hatchway, which forms an entrance into the observatory from the apartments below, which folds down, and forms a portion of the floor.
figure 91.
figure 91.
In the perspective view of the building fronting the title-page, the position and general aspect of the observatory-part of the building may be more distinctly perceived.
In public observatories, where zenith or polar distances require to be measured, it is necessary that there should be a dome, with an opening across the roof and down the north and south walls. Should an altitude or azimuth circle, or an equatorial instrument be used, they will require a revolving roof with openings and doors on two opposite sides, to enable an observer to follow a heavenly body across all the cardinal points. The openings may be about 15 inches wide, and the roof needs not be larger than what is requisite for giving room to the observer and the instrument, lest its bulk and weight should impede its easy motion. There have been various plansadopted for revolving domes. Fig. 92 represents a section of the rotatory dome constructed at East Sheen by the Rev. Dr. Pearson. This dome turns round on three detached spheres of lignum vitæ, in a circular bed, formed partly by the dome, and partly by the cylindrical frame-work, which surrounds the circular room of 9 feet diameter. A section of this bed forms a square which the sphere just fills, so as to have a small play to allow for shrinking; and, when the dome is carried round, the spheres, having exactly equal diameters of 4¼ inches each, when placedat equal distances from one another, keep their relative places, and move together in a beautifully smooth manner. These spheres act as friction rollers in two directions at the four points of contact, in case any obstacle is opposed to their progressive motion by the admission of dirt, or by any change of figure of the wood that composes the rings of the dome, and of the gang-way. No groove is here made, but what the weight of the roof resting on the hard sphere occasions. The dome itself moves twice round for the balls once, and has, in this way, its friction diminished. The wood of this dome is covered by Wyatt’s patent copper, one square foot of which weighs upwards of a pound; and the copper is so turned over the nails that fix it at the parts of junction, that not a single nail is seen in the whole dome. This covering is intended to render the dome more permanent than if it had been made of wood alone. At the observatory at Cambridge the dome is made chiefly of iron. In the figurea,arepresents one of the two oblong doors that meet at the apex of the cone, and a piece of sheet-copper bent over the upper end of the door which shuts last, keeps the rain from entering at the place of junction. The two halves of the dome are united by brass rods passing through the door-cheeks of wainscot ataandaby means of nuts that screw upon their ends, which union allows the dome to be separated into two parts when there may be occasion to displace it. The wooden platebb, which appears in a straight line, is a circular broad ring to which the covering wainscot boards are made fast above the eaves, andccis a similar ring forming the wall-plate or gang-way on which the dome rests and revolves.
figure 92.
figure 92.
figure 92*.
figure 92*.
Fig. 92* shows a small door that lies over the summit of the dome, and may be separately opened for zenith observations; the rod of metal with a ring at the lower end passing through it, serves to open and shut this door, and at the same time carries upon its upper end a large ball that falls back on the roof when the door is open, and keeps the door in a situation to be acted upon by the hook of a handle that is used for this purpose. The doorsaabeing curved, are made to open in two halves, the upper one being opened first, on account of its covering the end of the other; and the observer may open one or two doors as may best suit his purpose. The weight of this dome is such that a couple of wedges, inserted by a gentle blow between the ringsbbandcc, will keep it in its situation under the influence of the strongest wind.
It may not be improper to remark, that in all observatories, and in every apartment where celestial observations are made, there should, if possible, be a uniform temperature; and consequently a fire should never be kept in such places, particularly when observations are intended to be made, as it would cause currents of air through the doors and other openings, which would be injurious to the accuracy of observations. When a window is opened in an ordinary apartment where a fire is kept, there is a current of heated air which rushes out at the top, and a current ofcold air which rushes in from below, producing agitations and undulations, which prevent even a good telescope from showing celestial objects distinct and well defined; and, I have no doubt, that many young observers have been disappointed in their views of celestial phenomena, from this circumstance, when viewing the heavenly bodies from heated rooms in cold winter evenings; as the aërial undulations before the telescope prevent distinct vision of such objects as the belts of Jupiter, the spots of Mars, and the rings of Saturn.
An orrery is a machine for representing the order, the motions, the phases, and other phenomena of the planets. Although orreries and planetariums are not so much in use as they were half a century ago, yet as they tend to assist the conceptions of the astronomical tyro in regard to the motions, order, and positions of the bodies which compose the solar system, it may not be inexpedient shortly to describe the principles and construction of some of these machines.
The reason why the nameOrrerywas at first given to such machines, is said to have been owing to the following circumstance. Mr. Rowley, a mathematical-instrument-maker, having got one from Mr. George Graham, the original inventor, to be sent abroad with some of his own instruments, he copied it and made the first for the Earl of Orrery. Sir R. Steele, who knew nothing of Mr. Graham’s machine—thinking to do justice to the first encourager, as well as to the inventor of such a curious instrument, called it anOrrery, and gave Mr. Rowley the praise due to Mr. Graham. The construction of such machines is not a modern invention. The hollow sphere of Archimedes was a piece of mechanism of this kind,having been intended to exhibit the motions of the sun, the moon, and the five planets, according to the Ptolemaic system. The next orrery of which we have any account was that of Posidonius, who lived about 80 years before the Christian era, of which Cicero says, ‘If any man should carry the sphere of Posidonius into Scythia or Britain, in every revolution of which the motions of the sun, moon and five planets, were the same as in the heavens, each day and night, who in those barbarous countries could doubt of its being finished—not to say actuated—by perfect reason?’ The next machine of this kind, which history records, was constructed by the celebrated Boethius, the Christian Philosopher, about the year of Christ 510—of which it was said ‘that it was a machine pregnant with the universe—a portable heaven—a compendium of all things.’ After this period, we find no instances of such mechanism of any note till the 16th century, when science began to revive, and the arts to flourish. About this time the curious clock in Hampton Court Palace was constructed, which shows not only the hours of the day, but the motions of the sun and moon through all the signs of the zodiac, and other celestial phenomena. Another piece of mechanism of a similar kind is the clock in the cathedral of Strasburg, in which besides the clock part, is a celestial globe or sphere with the motions of the sun, moon, planets and the firmament of the fixed stars, which was finished in 1574.
Among the largest and most useful pieces of machinery of this kind, is the great sphere erected by Dr. Long in Pembroke Hall in Cambridge. This machine, which he called theUranium, consists of a planetarium which exhibits the motion of the earth and the primary planets, the sun, andthe motion of the moon round the earth, all enclosed within a sphere. Upon the sphere, besides the principal circles of the celestial globe, the Zodiac is placed, of a breadth sufficient to contain the apparent path of the moon, with all the stars over which the moon can pass, also the ecliptic, and the heliocentric orbits of all the planets. The Earth in the planetarium has a moveable horizon, to which a large moveable brass circle within the sphere may be set coincident, representing the plane of the horizon continued to the starry heavens. The horizons being turned round sink below the stars on the east side, and make them appear to rise, and rise above the stars on the west side, and make them appear to set. On the other hand, the earth and the horizon being at rest, the sphere may be turned round to represent the apparent diurnal motion of the heavens. In order to complete his idea on a large scale, the Doctor erected a sphere of 18 feet diameter, in which above 30 persons might sit conveniently, the entrance to which is over the South Pole, by six steps. The frame of the sphere consists of a number of iron meridians, the northern ends of which are screwed to a large round plate of brass with a hole in the centre of it; through this hole, from a beam in the ceiling, comes the north pole, a round iron rod about three inches long, and which supports the upper part of the sphere, to its proper elevation for the latitude of Cambridge, so much of it as is invisible in England being cut off, and the lower or southern ends of the meridians terminate on, and are screwed down to a strong circle of oak 13 feet diameter, which, when the sphere is put in motion, runs upon large rollers of lignum vitæ, in the manner that the tops of some wind-mills turn round. Upon the iron meridiansis fixed a zodiac of tin painted blue, on which the ecliptic and heliocentric orbits of the planets are drawn and the stars and constellations traced. The whole is turned round with a small winch, with as little labour as it takes to wind up a Jack, although the weight of the iron, tin, and the wooden circle is above a thousand pounds. This machine, though now somewhat neglected, may still be seen in Pembroke Hall, Cambridge, where I had an opportunity of inspecting it in November, 1839. The essential parts of the machine still remain nearly in the same state as when originally constructed in 1758.
The machine which I shall now describe is of a much smaller and less complex description than that which has been noticed above, and may be made for a comparatively small expense, while it exhibits, with sufficient accuracy, the motions, phases, and positions of all the primary planets, with the exception of the new planets, which cannot be accurately represented on account of their orbits crossing each other. In order to the construction of the Planetarium to which I allude, we must compare the proportion which the annual revolutions of the primary planets bear to that of the Earth. This proportion is expressed in the following table, in which the first column is the time of the Earth’s period in days; the second, that of the planets; and the third and fourth are numbers very nearly in the same proportion to each other.
figure 93.
figure 93.
On account of the number of teeth requiredfor the wheel which moves Uranus, it is frequently omitted in Planetariums, or the planet is placed upon the arbor which supports Saturn. If we now suppose a spindle or arbor with six wheelsfixedupon it in an horizontal position, having the number of teeth in each corresponding to the numbers in the third column, namely the wheel AM (fig. 93.) of 83 teeth, BL of 52, CK of 50, for the earth, DI of 40, EH of 7, and FG of 5; and another set of wheels moving freely about an arbor having the number of teeth in the fourth column, namely AN of 20, BO of 32, CP of 50—for the earth; DQ of 75, ER of 83, and FS of 148. Then, if these two arbors of fixed and moveable wheels be made of the size, and fixed at the distance here represented, the teeth of the former will take hold of those of the latter, and turn them freely when the machine is in motion. These arbors, with their wheels, are to be placed in a box of a proper size, in a perpendicular position; the arbor of fixed wheels to move in pivots at the top and bottom of the box, and the arborof the moveable wheels to go through the top of the box, and having on the top a wire fixed, and bent at a proper distance into a right angle upwards, bearing on the top a small round ball, representing its proper planet. If then, on the lower part of the arbor of fixed wheels, be placed a pinion of screw-teeth, a winch turning a spindle with an endless screw, playing in the teeth of the arbor, will turn it with all its wheels, and these wheels will turn the others about with their planets, in their proper and respective periods of time. For, while the fixed wheel CK moves its equal CP once round, the wheel AM will move AN a little more than four times round, and will consequently exhibit the motion of Mercury; the wheel EH will turn the wheel ER about1/12round, representing the proportional motion of Jupiter; and the wheel FG will turn the wheel FS, about1/29.5round, and represent the motion of Saturn, and so of all the rest.
figure 94.
figure 94.
The following figure (fig. 94.) represents theappearance of the instrument when completed. Upon the upper part of the circular box is pasted a Zodiac circle divided into 12 signs, and each sign into 30 degrees, with the corresponding days of the month. The wheel-work is understood to be within the box, which may either be supported by a tripod, or with four feet, as here represented. The moon, and the satellites of Jupiter, Saturn and Uranus, are moveable only by the hand. When the winch W is turned, then all the primary planets are made to move in their respective velocities. The ball in the centre represents the Sun, which is either made of brass or of wood gilded with gold.
By this Planetarium, simple as its construction may appear, a variety of interesting exhibitions may be made and problems performed, which may be conducive to the instruction of young students of astronomy. I shall mention only a few of those as specimens.
1. When the planets are placed in their respective positions by means of an Ephemeris or the Nautical Almanack, the relative positions of those bodies in respect to each other, the quarters of the heavens where they may be observed, and whether they are to be seen in the morning before sun-rise or in the evening after sun-set, may be at once determined. For example, on the 19th of December, 1844, theheliocentricplaces of the planets are as follows:—Uranus 2° Aries; Saturn 8° 27´ of Aquarius; Jupiter 7° 4´ Aries; Mars 12° 45´ Libra; the Earth 27° 46´ Gemini; Venus 29° 48´ Virgo; Mercury 7° 53´ Pisces. When the planets are placed on the planetarium in these positions, and the eye placed in a line with the balls representing the Earth and the Sun, all those situated to the left of the sun are to the east ofhim, and are to be seen in the evening, and those on the right, in the morning. In the present case, Uranus, Saturn, Jupiter, and Mercury are evening stars, and Mars and Venus can only be seen in the morning. Jupiter is in an aspect nearlyquartile, or 3 signs distant from the sun, and Uranus is nearly in the same aspect. Saturn is much nearer the sun, and Mercury is not far from the period of its greatesteasternelongation. Mars is not far from being in a quartile aspect,westof the sun, and Venus is near the same point of the heavens, approaching to the period of its greatestwesternelongation, and consequently will be seen before sun-rise as a beautiful morning star. Jupiter and Uranus, to the east of the sun, appear nearly directly opposite to Venus and Mars, which are to the west of the sun. The phase47of Venus is nearly that of a half-moon, and Mercury is somewhat gibbous, approaching to a half-moon phase. If, now, we turn the machine by the winch till the Index of the earth point at the 8th of August, 1845, we shall find the planets in the following positions:—Mars and Saturn are nearly in opposition to the sun; Venus and Mercury are evening stars at no great distance from each other, and Jupiter is a morning star. In like manner if we turn the machine till the Index point to any future months, or even succeeding years, the various aspects and positions of the planets may be plainly perceived. When the planets are moved by the winch, in this machine, we see them allat oncein motion around the sun,with the same respective velocities and periods of revolution which they have in the heavens. As the planets are represented in the preceding positions, Mercury, Jupiter and Mars, are evening stars, and Venus, Saturn, and Uranus, morning stars, if we suppose the earth placed in a line with our eye and the sun.
2. By this instrument, the truth of the Copernican or Solar system is clearly represented. When the planets are in motion, we perceive the planets Venus and Mercury to pass both before and behind the sun, and to have two conjunctions. We observe Mercury to be never more than a certain angular distance from the sun, as viewed from the earth, namely 27°; and Venus 47°. We perceive that the superior planets, particularly Mars, will be sometimes much nearer to the earth than at others, and therefore must appear larger at one time than at another, as they actually appear in the heavens. We see that the planets cannot appear from the earth to move with uniform velocity; for when nearest they appear to move faster, and slower when most remote. We likewise observe that the planets appear from theearth to move sometimesdirect, or from west to east, then become retrograde, or from east to west, and between both to bestationary. All which particulars exactly correspond with celestial observations. For illustrating these particulars there is a simple apparatus represented by fig. 95, which consists of a hollow wire with a slit at top which is placed over the arm of Mercury or Venus at E. The arm DG represents a ray of light coming from the planet at D to the earth at F. The planets being then in motion, the planet D, as seen in the heavens from the earth at F, will undergo the several changes of position, which we have described above, sometimes appearing to go backwards and at other times forwards. The wire prop, now supposed to be placed over Mercury at E, may likewise be placed over any of the other planets, particularly Mars, and similar phenomena will be exhibited.