Not ithstanding the great ingenuity displayed in these contrivances, we think that the reflecting cliche last de scribed, can neither be very accurate nor very pleasant in its use : the management of five clamps, which are to be made fast and loose in regular succession ten distinct operations to every pair of observations, any of which being mistaken or neglected would spoil a whole set, re quires a degree of attention not to be found in every ob ,erver. And the liability which there must be, even in the best work, to force the flying circles from their pla ces in clamping, and thereby to cause errors, without any means of detecting them, is a most forbidding cir cumstance. And should the index of the central mirror have a motion eccentric to those of the flying circles, the latter at every step will feel the effect, and it) consequence the result will be charged with the full error belonging to a single observation.
method of restoring the repeating property to his reflect ing eirel..ts, without in the least diminishing the advanta ges of the fixed frame. In this the circular border of the frame is not employed ; it has one flying deck, (which was probably borrowed from the instrument last described,) and it is divided in the usual manner. Two indices are employed ; one of them, below the circle, is screwed fast to the frame, the other is above, having, like the circle itself, free and indep,mdent motion round the centre ; the latter carries at the lower cud of the axis the index-glass. Both of the indices hay c screws for quick and slow motion, which act upon the flying circle ; and they are both furnished with opposite Vcrnicis. those of the lower index being brought round from below, in or der to count upon the divisions of the circle. When the index and horizon glasses are parallel, the two indices cross each other at right angles; therefore the upper one has a range of backward and forward motion through the full sextantal angle, without interfering with the lower One.
The effect produced by this arrangement is as follows. In an observation forwards, the circle being clamped by the fixed index, the moveable index advances upon the circle, and being clamped to it, the contact of objects is made by its proper screw. And in an observation back wards. the circle being released from the fixed index, is carried backwards along with the moveable one, reced ing upon the former, to N•hiell it is again clamped, and mein Lie contact of objects is made as before. These al ternate operations must be continued until a set is ob tained ; and then the upper index will give the amount of forward, and the lower Liat of backward observations.
With respect to repeating the angle, this instrument pro duces the same effect as Borda's circle ; but we cannot here come at the double result, as w e did in the arrange ment of Mendoza. in this latter, however, the clamp ing is performed after the contacts are obsers ed, which is its great defect ; but the former has no such impedi ment, the clamps being fixed before the contacts are made ; and therefore, in this respect, stands equal to a sextant, or any of the former constructions of the circle.
To such gentlemen as wish for a reflecting circle which repeats the angle, we hesitate not to recommend the one we have just endeavoured to describe ; and we have to inform the public, that they are indebted for the idea to Professor Hassler, a native of Swisserland, and a distinguished citizen of the United States of America.
Description and Use of the Portable Altitude and ?zinzuth Circle.
Ix our article ASTRONOMY, we gave to our readers the figure of an altitude and azi.,inth circle, with a short description ; and we there stated, that the instrument it self had been recently made for the observatory of Co lonel, now General, Brisbane. That instrument was constructed upon the model of the one used at West bury, by our present astronomer-royal ; and the obser vations which he there made with it, enabled him to give to the public that admirable catalogue of some of the fixed stars, which appeared in the Philosophical Trans actions for 1806. But as that instrument was of too large dimensions for convenient carriage, as the azimuth Since the above was written, we understand that there ac been suggested to Troughton an unexceptionable :xis was long and pointed downwards, greatly enlarging the hulk of package, and as it required a pedestal of peculiar structure ; however desirable these properties May be in an observatory, for which it was solely formed, it was rendered thereby quite Importable, and altogether inapplicable to geodetic purposes.
We have now before us, (Plate C XINI.) the figure of an instrument, which is free front the inconveniences above stated, the external appearance of which, we pre sume, will engage the attention of our readers in its fa vour. In confidence that this will be the case, we shall, without further encomium, proceed to describe its con struction and adjustment. We may, however, first pre mise, that in astronomy it is equally well adapted to the various purposes of the transit, quadrant, and equal alti tude instrument ; and in the field, it is capable of per forming the office of the most improved theodolite, and levelling instrument.