With the microscopes now in the diameter of the cir cle, and the vertical axis perpendicular, place the detach ed level upon the axis, in such a manner that it may touch nothing else ; now if it does not settle with the ends of the bubble at the divided scales, move the scales to the bubble, if it is within their range ; but if it is not, bring the bubble to the scales, when placed at equal dis tances from the middle, by means of the screw f, and its fellow on the opposite pillar. Then reversing the ends of the level, sec if the bubble settles at the indices ; if it does not, alter one half of the apparent error by f and its fellow, and the other half by the sliders of the level. The instrument will now give azimuths and right-ascensions, (that is, when placed correctly respect ing the meridian,) but without farther adjustment, zenith distances are only indirectly to be obtained from it.
To adjust it in this respect, we may proceed as fol.; lows: Point the telescope to any very distant object, which bisect by the middle horizontal wire, and read off by the microscopes the typarent zenith distance ; now reverse the instrument in azimuth, and turning the teles cope again upon the same object, bisect it as before, and again read off the angles which they shew. One of these will be an altitude, and the other a zenith distance. On one being reduced to the same name as the other, the mean of them will be the true zenith distance or altitude respectively ; and while the telescope bisects the object, the microscopes must be adjusted by their pro per screws, so as to shew that mean. This operation may be repeated for obtaining a greater degree of accu racy ; but its final determination should be deduced from observations upon many heavenly bodies, and the minute error that may remain unadjusted had better be allowed for.
The above are the grand adjustments of the circle be fore us; but there are many others which, to describe minutely, would require separate drawings. We do not, however, see the necessity for this ; for the general reader will, by what has already been said, come at all he can want—a general knowledge of its construction, properties, and use ; while those persons, who possess such an instrument, will, besides the above, have their attention guided to the parts of action; and the ingenious artist, who may attempt the making such instruments, by our description and figure, be enabled to pro ceed ; for his own resources ought to supply what may be hut partially represented. A brief enumeration, how ever, of some of the minor adjustments are here sub joined.
The microscopes of both circles have an adjustment for placing them in the exact diameters of their respec tive circles, which adjustment also sets them at their proper distance from the centres. By another adjust
ment, the whole length of the microscopic tube is varied, in order to make the magnified image of a division exactly measure the scale of five revolutions of the mi crometer screw. A consequent adjustment of back ward or forward motion of the whole tube restores dis tinct vision, which was necessarily lost by the former operation. The five single minutes are indicated by as many notches, seen at the side of the field of view of the microscopes ; they are indented by the micrometer screw itself, and are adjusted to their place by a screw at the end of the box, opposite to the micrometer head, acting upon the plate which contains them. By these means, a revolution of the screw becomes the exact measure of a minute ; and it only requires the head of that screw to be divided into 60 equal parts, in order to give seconds. This circular scale of seconds has a motion round the head, whereby its zero may be brought to the pointer, without moving the cross lines within.
Both the levels are constructed without adjusting screws, the moveable divided indices answering the same purpose ; for if the point of adjustment, as ascer tained by reversion, be within the range of the bubble, the performance will be equally accurate, as it would have been, had that point been in the middle of the tube.
It may be remarked, that, after the principal adjust ments have been carefully made, they should not be dis on slight occasions. If, therefore, the horizontal axis were occasionally found to be out of level, to the amount of a second or two, it would be wrong to adjust it by the screw f or its fellow, because that would affect the circle respecting its reading microscopes. Such a small deviation would be best corrected by touching one or more of the feet screws.
Of the instrument which we are now about to dismiss, it has been said, that its principal telescope magnifies 60 tunes, and that the angles are read to single seconds ; and we may add, that the circles being divided by an accurate engine, the work of which is estimated to be within 5" of the truth, and the levels, by which the verifications are made, being ground to a long scale, so as to be depended on to less than 2", reasonable expec tations may be formed, that when used under favoura ble circumstances, two observations, in reverse positions with it, will give the place of an object in the heavens, in declination within 5", and in right ascension within 0.5" of tune. Nor can much of this degree of accuracy be abated, where, in surveying, land objects are observed ; hut much depends here upon what kind of objects pre sent themselves for bisection.