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its removal, but serves at all times as a check upon its adjustment. The quadrant is likewise furnished with a Vernier, and all that is necessary for reading off the arcs with great accuracy, and which may be readily done to 10ths of seconds. The adjustment of the instrument requires some explanation.
In these adjustments, the first thing to be done is to bring the axis to a vertical position: when the plumb-line is employed for this purpose, turn the quadrant horizontally till the telescope becomes parallel to a line joining any two of the feet by which the whole is supported : then turn one of the foot screws till the wire bisects the lower dot, and with the proper screw bring the upper dot to the wire. When this is done, reverse the telescope by turning the instrument 180° in the azimuth; and then, if both dots are again bisected, the axis is vertical in reference to that direction in which the telescope pointed. The telescope is next to be turned so as to point to the third foot of the tripod, and the wire made to bisect the lower dot by turning the proper screw; and it will also be found to bisect the upper dot, if the first adjustment was properly made: but if it does not, the operations must be repeated till both the dots are bisected in all the reversed positions of the instrument, when the pedestal will be in the proper vertical direction.
A second important adjustment is that which relates to the collimation of the telescope, by which it is made parallel to the line that passes from the centre of the quadrant to zero on the limb, at the same time that zero on the vernier coincides with zero on the quadrantal arc. Various methods have been proposed for accomplishing this; but the two following, which serve as effectual checks to each other, will be sufficient; they will also serve at the same time to detect any error, should there be any, in the total arc. The one of these relates to the vertical, and the other to the horizontal radius of the quadrant. To adjust the vertical line :-the axis of the quadrant being brought into the perpendicular by the method already described; fix upon a star near the zenith, when exactly on the meridian, and measure the altitude by the cross-wire in the field of view in the usual way, and note the result. If these readings, the one on the quadrantal arc, and the other on the excess of that arc beyond 90°, prove to be at equal distances from the point 90°, the horizontal wire is truly placed; but if not, half the difference of the readings must be corrected by means of the proper screw for raising or depressing the wire. This may be done by directing the cross-wire to a distant mark till it bisects it; and then moving the screw of the slow motion of the vernier half the quantity required, and by bringing back the crosswire thus displaced to its original mark: a repetition of the operation will place the cross-wire in such a situation, that zero on the vernier will be in its proper position with respect to the point 90°; or, when the half difference is ascertained, it may remain as an error of adjustment, to be applied with the sine plus or minus, as a correction of future observations, as circumstances may render it-necessary.
To adjust the instrument by means of the horizontal line passing through the zero on the limb of the quadrant, a second telescope is required : this should turn on pivots at the back of the quadrant, and be on a level with the said horizontal line. This may with propriety be called the adjusting telescope, and may be directed to any distant mark, both before and after an altitude is taken, for the purpose of detecting any deviation in the vertical position of the axis that may take place during the operation. It is necessary first to ascertain if this telescope be properly fixed : for this purpose make it bisect some distant object by its cross-wire, and then after turning it
half round, if the object be again bisected by the same wire, the telescope is truly fixed. If the mark is not cut by the wire in the reversed position of the telescope, another, either higher or lower, as the case may require, must be chosen, and the cross-wire made to bisect it when the telescope is reversed, by turning the proper screws: it will thus be adjusted for collimation. The zero on the vernier must next be brought to coincide with the zero on the limb, and the telescope of observation directed to the distant mark by which the adjusting telescope had been brought to its true position. This mark being bisected by both telescopes, while the level and the plumb-line show that the vertical axis is perpendicular, turn the quadrant half round its azimuth, and reverse the adjusting telescope, so as to see the same mark again; and if it be found to cut it as before, the horizontal line of the quadrant will be correct, and the whole arc without error, supposing the telescope of observation to have been previously adjusted for collimation by the point 90°, as above described.
In these adjustments it has been supposed that the cross-wire was perfectly horizontal, or that the parallel lines were perpendicular to the horizon. This, however, is easily proved; for by directing the telescope to a fine distant mark, if one of the vertical wires continue to bisect it through the whole field of view, while the telescope is elevated or depressed, the position of the wires is correct; but if not, they must be brought to their proper position by turning the screws designed for that purpose. In all good instruments, the plane of the quadrant should be parallel to its axis of motion, and the line of collimation of the telescope should also be parallel to that plane.
(To be continued.]
The Naturalist's Diary
For APRIL 1823.
Thou sendest forth Thy spirit, they are created; and Thou renewest the face of the earth.-PSALM civ, 30.
During the death and the darkness of winter, we walk forth, as into an unbounded burial ground: the nicest eye, the most minute investigation, cannot, in many instances, discover one single trace of all that profusion of vegetable and animal life which so lately figured and flourished. The leaf, and the fruit, and the stem, and the very root of the plant, are gone; and the future being, not only of the individual insect, but of the race itself, rests upon an existence so minute and diminutive, as to elude the very closest observation. But, the spirit of renewal being sent forth, the Sun having advanced in his annual circuit, dispersing light and heat, and calling up life and joy, in his progress;—the fields and the forests, over which winter had brooded in death and in darkness, rejoice, and swell, and freshen, and bud, and burst into life.-There is a resurrection from the dead,-and the air, and the earth, and the water, are peopled with inhabitants, who have apparently shared in the benefit of so general a revival.
It is in SPRING—more audibly, perhaps, than during any other season, that the voice of God comes down upon us, and comes up around us,-and breaks in through all the accessible parts of our moral apprehensions, in tones of irresistible and sustained endearment. It is in spring that Divine goodness walks forth unveiled--that the pastures of the wilderness do spring that the fig-tree and the vine put on their promise—that the Spirit of renewal is sent forth,-and a general expression of cheerfulness prevails. It is in spring that the foot which presses the soil, falls more lightly,-while the song of love and of joy is borne up, on the wings of harmony, even to
heaven's gate. It is in spring, that, aided by association, combination, and reflection, man is enabled to convert the visible features of beauty, the varied and pleasing attitudes of nature, into an inexhaustible source of enjoyment'.
The changeful' character of the weather in the month of April, sometimes mild, with gentle showers, and occasionally frosty, has been noticed by most of our poets: it is thus prettily described by Chamberlayne, one of the bards of England's good 'olden tyme:
Look how a bright and glorious morning, which
Herself, and weeps for sorrow. But April, like human life, has her splendid lights, as well as her deepening shadows; and
the darkest day Live till to-morrow will have passed away. In the language of Mr. WORDSWORTH, with some of whose exquisite Sonnets we have enriched our pages,
Now the storm hath ceased, the birds regain
To the blue ether and bespangled plain. In our last year's Diary for May, we introduced some observations on the appearances of Spring in Persia: pass we now to Sweden, and let us contemplate the beautiful description of her Spring, as given by Mr. Richter:- A Swedish Spring,' he observes,
which is always a late one, is no repetition, in a lower key, of the harshness of winter; but anticipates, and is a prelibation, of perfect summer,laden with blossoms-radiant with the lily and the rose; insomuch, that a Swedish summer night re
The Seasons contemplated in the Spirit of the Gospel, by the Rev. T. Gillespie. (Cadell, 1822.)