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—the height of a cloud, by observation on its altitude and velocity—and numerous other problems, the solution of which depends upon angular measurements. A Mural Quadrant is 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much used in astronomical observations, yet it may be proper to state, that its use has now been almost completely superseded by the recent introduction of Astronomical Circles, of which we shall now give the reader a very short description, chiefly taken from Troughton’s account of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.
THE ASTRONOMICAL CIRCLE.
figure 89.
An astronomical circle is a complete circle substituted 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 means of the micrometer microscopes mm, 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 screws cc, and the space moved through is ascertained by the divisions on the graduated head above c, 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. At h is represented a level for placing the axis in a true horizontal line, and at k is 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 bar f. 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 at e, screwed to the ring i, 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 screw d to incline the axis to the east or west, while the screw b gives the upper one i a 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 piece s, 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 ring i, and the microscopes themselves are adjustable by screws, to bring them to zero and to the diameter of the circle. A little above the ring i is 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 screw a, 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 tube n, 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 water t, in order to check its oscillations, and is supported on a shelf proceeding from one of the pillars. At the lower end of the tube n are fixed two microscopes o and p, 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 screws d, 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 screws d b, 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 that greater 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 to bad 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 complicated pieces 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.