CIRCLE. 



fleeting 

 circlr. 



I*i* it 



an altitude of the iun with ihe tea horizon, can be taken 

 " backward* ; beciute the dirk -glasses at that angle pre- 

 :he reflected rtyt of light from falling on the inlex- 

 (Us* I whence it becomes necotaiy, when the angle to 

 be taken it quite unknown, to observe forward* first, 

 where the whole range ii without interruption ; v 

 ai, in that backward*, you will Ion- tight of the reflect- 

 ed image about that angle. But in uch distances where 

 the tun it out of the ouettion, and when hit altitude it 

 taken with an artificial hori/on, (the thade being applied 

 to the end of the telescope,) that angle may be measu- 

 red nearly as well at any other j for the ray incident on 

 : dex-glat* will pan through the transparent half of 

 the horizon. glatt, without much diminution of their 

 brighton*. 



The advantage! of thi* instrument, when compared 

 with the KStant, are chiefly these : The observation* for 

 finding the index error are rendered uteless, ail know- 

 ledge of that being put out of the question, by obser- 

 ving both forwards and backward*. By the same means 

 the error* of the dark glattet are also corrected ; for, if 

 they increase the angle one way, they must diminish it 

 the other way by ihe tame quantity. Tliit also perfect- 

 ly correcti the errors of the horizon glass, and those of 

 the index -glass very nearly. But what is still of more 

 contequcncr, the error of the centre is perfectly correct- 

 ed, by reading the three branches of the index ; while 

 thit property, combined with that of observing both ways, 

 probably reduces the errors of dividing to one -sixth part 

 of their simple value. Moreover, angles may be mea- 

 sured as far as one hundred and fifty degrees, consequent- 

 ly the sun's double altitude may be observed when his 

 distance from the zenith is not lest than fifteen degrees ; 

 at which altitude, the head of the observer begins to in- 

 tercept the rays of light incident on the artificial hori- 

 zon | and, of course, if a greater angle could be mea- 

 sured, it wojld be of no use in this rcbpect. 



...... -. :. --:-:.:, requires 



three adjustment*. First, the index-glass perpendicular 

 to the plane of the circle. Thin 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 see that part of the limb 

 which is next you reflected in contact with the oppo- 

 site tide 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 per- 

 formed by a capstan-screw, at the lower end of tlit- frame 

 of that glass; and is known to be right, when, by a 

 sweep with the index, the reflected image of any object 

 will past exactly over, or cover the image of that object 

 teen directly. The third adjustment is, for maki. ^ tlic 

 line of coUimation parallel to the plane of the circle. 

 Thit it performed by two small tcrewt, which also fas- 

 ten the collar into which the telescope screws, to the up- 

 right stem on which it in mounted : this is known to be 

 right, when the tun and moon, having a distance of one 

 hundred and thirty degrees, or more, their limbs arc 

 brought in contact, just at the outside of that wire which 

 it next to the circle j and then, examining if it be the 

 same, just at the outtide of the other wire : itt being to 

 U the proof of adjustment." 



j Plate CXLV. was chiefly designed to represent the 

 stand, whereon the British construction of the n Mi cling 

 circle hat been frequently mounted for making observa- 

 lions OD shore, such as, by the aid of an artificial hori- 

 ton, we obtain altitudes of the heavenly bodies for the 

 latitude or time, as well at by the lunar observation!, to 



find the longitude on Ltnd. This perspective view was Circle. 

 also inlvndrd lr exhibit such parts of the instrument as " ""Y"" 

 are not sufficiently shewn in tlio other Figures. 



At the bottom ot the PI. tr, wo see the tripod of the 

 t:n:l is supported by three linger screws, the use of 

 which is not for placing the instrument perpendicular, 

 which is their usual office ; they are 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 alto give that small mo- 

 tion which is required to make the images pats each 

 other, for ascertaining the points of objects which ought 

 to be brought into contact. .ewe thus prc - 



duce, in the simplest and i.eate-t manner, the full effect 

 that could be gained, were all the motion* ot 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 -A iiirh 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 a' 

 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 

 thank of a screw passes ; and there U 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 the free use 

 of both his hands ; one of which is always wanted to 

 act upon the index-screw, while the other is required at 

 the screws of the tripod ; and, at the very moment of 

 contact, 'houid perhaps always be employed at the eye- 

 end of the telescope, touching also the face of the ob- 

 server, and thereby preventing the wandering of the eye. 

 Thus mounted, the reflecting circle becomes of itself a 

 littl.- observatory, and enables a skilful observer to turn 

 it to good account in the sciences of astronomy, geogra- 

 phy, and navigation. 



1'h 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 5"; where the reflecting glasses and 

 telescopes are good, and power considerable (about 1'J), 

 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 are of opinion that 

 a scries 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, &c. of Mayer, as improved 

 by Mason, were used for the construction of the Nauti- 

 cal Almanack, the sextant, at it came from the hands f 



