CIRCLE. 



the filed index, the moveable index advance* upon the 

 "V*"' circle, and being clamped to it. the contact of object* i* 

 made by it* proper (crew. And in in observation back- 

 wards, the circle being released from the fixed index, is 

 carried backward* along with the moreable one, receding 

 upon the former, to which it i* again clamped, and then 

 the contact of object* i* made a* before. These alternate 

 operation* mutt be continued until a tet i* obtained; and 

 then the upper index will giro the amount of forward, 

 aad the lower that of backward observations. With re- 

 spect to repeating the angle, thi* instrument produces 

 the tame effect a* oorda'i circle ; but we cannot here come 

 at the double result, a* we did in the arrangement of 

 Mendoza. In thi* latter, however, the clamping is per- 

 formed after the contact* are observed, which is its great 

 defect ; but the former ha* no such impediment, the 

 clamp* being fixed before the contacts are made ; and 

 therefore, in thi* respect, stands equal to a sextant, or 

 any of the former construction* 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 

 Profc*sor Hassler, a native of Swisserland, and a distin- 

 guiihed citizen of the United States of America. 



CHAP. II. 



Description and Use of the Portable Altitude and 

 Azimuth Circle. 



SECT. I. Detcription of the Altitude and Azimuth Circle. 



IN our article ASTRONOMY, we gave to our readers the 

 figure of an altitude and azimuth circle, with a short de- 

 mit "' *cription ; and we there stated, that the instrument it- 

 cirdr. **tf nt d 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- 

 vation* which he there made with it, eaabled 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 

 axis was long and pointed downwards, greatly enlar- 

 ging the bulk 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 form- 

 ed, it was rendered thereby quite unportable, and alto- 

 gether inapplicable to geodetic purposes. 



LT We have now before us, (Plate CXLVI.) the figure 



CXLVI. of an instrument, which is free from 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 rqual alti- 

 tude instrument ; and in the field, it is capable of per- 

 forming the office of the most improved theodolite, and 

 .e veiling instrument. 



The figure bears the date of IS 10, the year in which 

 the circle it was drawn from was really made ; but 1792 

 is the date of it* first production by the artist, whose 

 Uroe it bears. 



Beginning our description at the bottom, the part BB 



represent* a strong tripod, the feet-screws of which, Circle. 

 A, A, with milled heads downwards, support the whole, V "~V"' 

 and enable it to stand upon a table, window scat, or stone 

 pillar, as circumstances may require j its own packing- 

 box is also fitted up for a pedestal, arid serves in cases 

 where a better cannot be provided. Immediately above 

 the tripod, and in close contact with it, is the azimuth 

 circle CC; centered upon a vertical axis, which is strong. 

 ly united with the tripod at its centre. This circle is 

 connected with the tripod, not only by the axis round 

 which it turns, but also at three places near its border, 

 but in such a way as to admit of a motion of three or 

 four degrees only, wholly governed by a slow-moving 

 screw, the head of which is seen below to the right. Be- 

 low, and on the left, appears the eye-end of a telescope 

 R, which is furnished with the means of vertical and ho- 

 rizontal motion. It is fixed to the circle, (not the tri- 

 pod.l and partakes of the same motion with it, when 

 acted on by the slow-moving screw. Round the vertical 

 axis already mentioned, on the conical socket D, revolves 

 the azimuth index, near to the circle, but yet without 

 touching it. This index is, like the circle, an entire 

 plate, deep for the sake of strength, and hollow for the 

 sake of lightness ; the edge terminates in a half contratc 

 border, as appears at E. The index may be clamped 

 fast to the circle by a screw, the head of which is slu-wu 

 at a, when another, represented at b furnishes a gradual 

 slow motion. Two micrometer-microscopes FF, for 

 subdividing and reading the divisions of the circle, arc 

 well secured to the index by three pillars each. Their 

 positions are on directly opposite points of the index 

 with which they revolve. 



At right angles with the microscopes, are erected upon 

 the index two stout pillars GG, the use of which is chief- 

 ly to sustain the weight of the upper part of the instru- 

 ment, and to render steady the ends of the transit axis. 

 These pillars arc connected by a piece T, with the soc- 

 ket of the azimuth axis near its upper end, by which 

 means the pillars and socket are made mutually, to sup- 

 port each other. 



The principal part of the weight of the upper works 

 is made to bear upon the tops of the pillars, and is sus- 

 tained by a friction- wheel at the centre of each, as at g. 

 These wheels are urged upwards by a spiral spring, the 

 force of which is nearly equivalent to the incumbent 

 pressure. On receiving the weight, the wheels retire 

 within the pillar, and allow the different parts to find 

 their proper bearings. 



For the purpose of supporting a horizontal axis 16 

 inches long, which is nearly twice the distance of the pil- 

 lars, a strong piece, as at f, projects outwards horizon- 

 tally from each, and to these are affixed the adjustable 

 angles, in which the ends of the axis rest, as shewn at d. 

 The horizontal axis H, thus supported, form* the central 

 part of the altitude circle 1III. This circle is double, each 

 part being well connected with the axis, and as firmly uni- 

 ted to the other, by short pillars placed perpendicularly 

 between them. The divisions of this circle, as the one be- 

 low, are degrees subdivided to every five minutes, and 

 like them are read by micrometer-microscopes, which, 

 again subdivide to every single second. The microscopes 

 are attached to the extremities of two bent conical tubes 

 NN, which arc screwed fast to one of the great pillars : 

 the microscopic part is denoted by SS, and tin- scales, 

 which exhibit the subdivisions, are represented at h, h. 

 The circle is acted on by a quick and slow moving appa- 

 ratus K, which is made fast to the opposite pillar, as shewn 

 at G. It is so formed as to oppose a stout resistance in 

 the direction of the plane of the circle, but in the line of 



