This instrument, in common with the sextant, requires three adjustments. First, the index-glass perpendicular to the plane of the circle. This being done by the ma ker, and not liable to alter, has no direct means applied to the purpose : it is known to be right, when, by look ing into the index-glass, you sec that part of the limb which is next you reflected in contact with the oppo site side of the limb, as one continued arc of a circle : on the contrary, when the arc appears broken, where the reflected and direct parts of the limb meet, it is a proof that it wants to be rectified. The second is, to make the horizon-glass perpendicular. This is perform ed by a capstan-screw, at the lower end of the frame of that glass ; and is known to be right, when, by a sweep with the index, the reflected image of any object will pass exactly over, or cover the image of that object seen directly, The third adjustment is, for making the line of collimation parallel to the plane of the circle. This is performed by two small screws, which also fasten the collar into which the telescope screws, to the upright stem on which it is mounted: this is known to be right, when the sun and moon, having a distance of one hun dred and thirty degrees, or more, their limbs arc brought in contact, just at the outside of that wire which is next to the circle ; and then, examining if it be the same, just at the outside of the other wire : its being so is the proof of adjustment." Plate CXLV. was chiefly designed to represent the stand, whereon the British construction of the reflecting circle has been frequently mounted for making observa tions on shore, such as, by the aid of an artificial hori zon, we obtain altitudes of the heavenly bodies for the latitude or time, as well as by the lunar observations, to find the longitude on land. This perspective view was also intended to exhibit such parts of the instrument as arc not sufficiently shewn in the other Figures.
At the bottom of the Plate, we see the tripod of the stand is supported by three finger screws, the use of which is not for placing the instrument perpendicular, which is their usual office ; they arc here- designed to bring the objects under observation, exactly into the centre of the field of view, after having been brought nearly thither by hand. They also give that small mo tion which is required to make the images pass each other, for ascertaining the points of objects which ought to be brought into contact. These screws thus pro duce, in the simplest and neatest manner, the full effect that could be gained, were all the motions of the stand governed by adjusting screws. A vertical pillar is in serted into the centre of the tripod, and unscrews there for the sake of snug packing. At the top of the pillar, and within it, there is an axis whereon the instrument turns round horizontally : this motion respecting stiffness being regulated by a finger screw at top. A cross piece above the pillar, to which it is attached, bears at its ends, about four inches apart, two upright supporters, in which is centered an axis at right angles to the for mer. The motion round this axis may also be clamped
or set free, by two finger screws at its opposite ends. By these two motions, the circle may be placed in any position that observations can require. Two counter poising cylinders, whose connecting levers apply at the ends of the horizontal axis, keep the instrument at rest in all positions. There is a socket fixed at right angles to the middle of the horizontal axis, through which the shank of a screw passes; and there is a cock firmly fas tened to the centre of the circle, which receives the tap ped end of the screw, just clear of the index-glass, bind ing the circle and stand strongly together, yet allowing a free motion of the former in its own plane round the shank of the screw as an axis.
The three motions of the stand above described are all essential ; for with fewer the varied compound mo tion of the hand cannot be imitated. Steadiness in a stand for a reflecting instrument would be a property of no great value ; for its only use is to relieve the arm of the observer from the weight, and allow him thc free use of both his hands; one of which is always wanted to act upon the index-screw, while the oth6r is required at the screws of the tripod ; and, at the very moment of contact, should perhaps always be employed at the eye end of the telescope, touching also the face of the ob server, and thereby preventing thc of the eye. 'flips mounted, the reflecting circle becomes of itself a little observatory, and enables a skilful observer to turn it to good account in the sciences of astronomy, geogra phy, and navigation.
The greatest error to which dividing by a good en gine is liable, we think may be taken at about 20"; the six readings required in a double observation, on differ ent parts of the circumference, will probably reduce that error to within where the reflecting glasses and te lescopes are good, and power considerable (about 12), a mean of contacts will come out within this quantity. And where every other source of error is corrected by the principles of the instrument, we arc of opinion that a series of lunar observations will give the longitude on land nearly, if not quite as accurately, as can be obtain ed from an occultation of a star by the moon, when ob served with a powerful telescope.
When the Lunar Tables, Cc. of 'Mayer, as improved by Mason, were used for the construction of the Nauti cal Almanack, the sextant, as it came from the hands of our best makers, was, in point of exactness, nearly on a level with its columns : but the Tables of Gauss, com puted from the theorems of La Place, left both far be hind. These now, however, form the basis of our Alma nack, and an instrument of commensurate exactness is to be found in the British circle.
With the reflecting circle so constructed, the Alma nack so improved, and with Captain Alendoza's excellent Tables, which far surpass every thing of the kind seen before, finding the longitude, either at sea or land, is no longer either a doubtful or a difficult operation.