CIRCLE. 



493 



Circl*. the axis it is extremely flexible. The telescope ZZ passes 

 -Y through the central part of the horizontal axis, to which 

 it is fastened ; it is also united to the double circle, be- 

 tween the parts of which it is situated. A lantern, not 

 represented in the Figure, applies to one of the large pil- 

 lars, opposite to the end of the axis, on the reverse side, 

 from which light is directed through the axis, perforated 

 for that purpose, and falling upon a plane covered with 

 gold-leaf, is from thence reflected to the eye, for the pur- 

 pose of illuminating the field of view of the telescope. A 

 level, as shewn at X, hangs upon two pivots fixed to the 

 crooked supports of the microscopes, and occupies a po- 

 sition between the pillar and divided side of the circle : 

 one of its pivots is adjustable by the finger-screw r. The 

 level has sliding ivory scales, which indicate to a second 

 the position ofthe parts with which it is connected. A 

 similar level, not represented in the Figure, applies occa- 

 sionally to the pivots of the horizontal axis, upon which 

 it is made to rest when in use, having been previously 

 passed through between two of the radii of the double- 

 framed circle. This has also sliding divided scales, and, 

 like the other, indicates single seconds. 



The instrument above described is of moderate dimen- 

 gioni, the diameter of the vertical circle being 18 inches, and 

 that ofthe horizontal one 15 inches. They are both di- 

 vided by lines (not dots) into spaces of five minutes, and 

 the pans actually divided are, by their respective micro- 

 meters, subdivided and read, as was said above, to every 

 second. The essential part of those micrometers consists 

 of two wires, which cross each other like an X, and are 

 moved along by a fine screw, in the direction of a tan- 

 gent, to that part of the circle which may be opposed to 

 them. When the cross is at the zero of its scale, it is the 

 index of the divisions of the circle, and the quantity to 

 be measured by the motion of the cross is that by which 

 the next dividing line has passed beyond that zero, and 

 is always additive. The general construction of this mi- 

 crometer is not materially different from the parallel-line 

 one, which was described in our article ASTRONOMY. 



The length of the uppr r telescope is 30 inches, the 

 aperture of its object-glass 2 inches, and iti magnifying 

 power about 60. It is indeed furnished with eye-piece* 

 of both less and greater power than this, which may have 

 their use in particular cases ; but in general, this is reck- 

 oned the most useful power for such a telescope. The 

 lower telescope is only about 18 inches in length, and 

 its aperture and power proportionable. The whole height 

 ofthe instrument is something more than three feet, and 

 its weight is about 75 pounds. 



In the eye-piece of the upper telescope are placed five 

 vertical and five horizontal lines, the angular distance of 

 which is about 6' 24" of a degree; this distance, when 

 the sun's diameter is 32 minutes, allows an equal portion 

 of time between all the ten contacts in observing his tran- 

 sit, or passage over the meridian. The five horizontal 

 lines are for the purpose of observing equal altitudes, and 

 when the diameter of that luminary is as above, will, 

 without moving the circle, afford ten distinct sights, and 

 these also will be equal to each other. 



The lower telescope has in its focus only two cross 

 wires. It is never elevated to the heavens ; its only pur- 

 pose being to keep match, to as to shew if the whole in- 

 strument preserves its position unvaried respecting some 

 distant object, while a set of observations is carried on. 



In the eye-end of the upper telescope there are several 

 adjustments. One of these is for placing the cross-lines 

 in the focal point of the object-glass ; and this is effected 

 by drawing out or pushing in the whole of the eye piece 

 respecting the main tube. The whole eye-piece also ad- 



mits of a limited motion round the axis of -vision, where- 

 by the cross-lines are adjusted respecting their vertical 

 and horizontal position. The plate in which the cross- 

 lines are inserted, is acted on horizontally by two oppo- 

 site screws ; they draw, and produce that adjustment, 

 which places the intersection of the middle lines perpen- 

 dicular to the horizontal axis. When this is verified by 

 reversing the circle upon its angular supports, the line of 

 collimation of the telescope will describe a great circle 

 round the axis on which it revolves. 



The cross-lines of the lower telescope are also acted 

 on by two screws, like the upper one ; but these are for 

 no other purpose than for bisecting an object with the 

 greatest accuracy, which cannot be done by the motions 

 of the telescope itself, because they are not governed by 

 adjusting screws. 



To proceed in our account of the adjustments of this 

 instrument, we find the feet-screws ready for placing the 

 vertical axis truly perpendicular. This they will do by 

 help ofthe level X, which having its bubble brought to 

 the divided scales in any position, and then the instrument 

 being turned half round in azimuth, an error will appear 

 the double of that by which the axis is inclined ; one half 

 therefore must be corrected by the feet-screws, and the 

 other by the screw r which acts upon the level. Thi 

 will set the axis perpendicular in the direction of the le- 

 vel; and now, the instrument being turned through a 

 quadrant, and the level brought by the feet-screw again 

 to the sliders, this adjustment will be effected. The first 

 part of the operation is best performed with the level in 

 the direction of two of the feet-screws, because the se- 

 cond may then be made by the other screw without dan- 

 ger of deranging the first. It need scarcely be mention- 

 ed, that the operation should be repeated for greater ex- 

 actness ; nor that, when the adjustment is perfect, the 

 bubble of the level will preserve its centrical position, 

 while the instrument is turned quite round in azimuth. 

 By this adjustment, the telescope is made to describe lines 

 parallel to the horizon. The reading microscopes may 

 now be adjusted to their places, so as to have their zeros 

 exactly in the diameter of the azimuth circle. 



The next great adjustment is to place the vertical cir- 

 cle at such a height, that its two microscopes shall be 

 directed to two opposite points in its horizontal diameter. 

 This is done by the screw J', which, acting upon the an- 

 gular bearings of the horizontal axis, carries it up or 

 down, as may be required. The farther pillar has the 

 same kind of screw, which acts in like manner upon the 

 support of the opposite end of the axis. It must here 

 be mentioned, that through an inadvertence of the ma- 

 ker, the draftsman had access to the instrument, at a time 

 when the angular bearings d were screwed almost out of 

 their sockets e, on which account the socket and friction- 

 wheel g appears to lift the circle too high ; and the drafts- 

 man, knowing that the microscopes should occupy the 

 line of the horizontal diameter of the circle, added that 

 part in the tubular support which is seen between the cir- 

 cular rings, which part was not to be found in the instru- 

 ment. 



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 distance! 

 from the middle, by means of the screwy, and its fellow 

 on the opposite pillar. Then reversing the ends of the 

 level, see if the bubble settles at the indices ; if it doei 



Circle. 



