r i R c L i:. 



Mayer'* 



irflrcting 

 rclc. 



Purr 

 iM.I V. 

 fig. i. 



clr, we Uuil endrtroor to trace itt progress through the 

 principal improvement* which it hit undergone! from it* 

 tint invention down to the present time. 



CHAP. I. 

 On Ike Rrfecting Circle. 



Tut instrument which bean the appellation of reflec- 

 ting circle, is of the nature of Hadley'i quadrant, and is 

 built upon the nme foundation ; and, for the present, 

 we shall consider our reader* a* acquainted with that 

 well known instrument, a full account of the principles, 

 properties, and ute of which, ihall be reserved, in order 

 to be introduced under it* appropriate title. 



Although there are few purposes in practical astrono- 

 my and surveying, to which this kind of instrument may 

 not be advantageously applied, yet it was originally con- 

 structed fur, and ha* been chiefly used in making lunar 

 observations for finding the longitude at sea. Tlu- Lite 

 Professor Mayer of Gottingen, the ingenious and labo- 

 rious astronomer who first furnished tables for that pur- 

 pose, teems to have judged well of what kind of instru- 

 ment would ultimately be used, to effect the best prac- 

 tical solution of the lunar problem. Agreeable to this 

 idea we fn.d, that when he transmitted to England his 

 solar and lunar tables, they were accompanied by draw- 

 ing*, and a description of the first reflecting circle, and 

 were published by the British Board of Longitude in the 

 year 1770. 



A* it is not our intention to extend this article to an 

 unnecessary length, we shall, without farther introduc- 

 tion, proceed to a description of Mayer's instrument. 



SECT. I. Detcription of Mayer's Reflecting Circle. 



Fig- ' of Plate CXLI V. is a plan of this circle. Its 

 chief part i* a cross, surrounded by a circular limb all in 

 one piece. Immediately above this is an index A, to 

 which is affixed the telescope and horizon glass. Still 

 higher, another index B carries the central glass. Both 

 A and B move freely round upon a pin fixed in the cen- 

 tre of the circle, quite independent of each other, and 

 both of them apply close to the border of the circle, for 

 the purpose of reading off its divisions. From the form 

 of the exterior ends of the indices, one would imagine 

 that the limb was intended to be divided by diagonal 

 lines, but a* the vernier was known at that time, it is 

 probable, that no instruments were ever constructed in 

 this respect like the figure. At the time the circle was 

 proposed by Mayer, the art of dividing instruments was 

 in a very rude state, and to render bad dividing of little 

 or no consequence, was the aim of this particular con- 

 struction. 



It will be presently seen, that, with this circle, as many 

 observations as we please may be successively made from 

 point to point upon the limb, without reading off any 



ri ii ii . . ^ ' 



more of them than the accumulated angle, having previ- 

 ously indeed noted the point at which we commenced. 

 This is a very different thing from taking many observa- 

 tions with a sextant or quadrant, when the angles that 

 compose a set being all read off nearly upon the same 

 pan of the arc, will most probably be all affected with 

 nearly the same error. It is also very different from ma- 

 king many unconnected observations with a circle, where 

 the beginning and end are read off in each ; for, in this 

 case, every angle will be charged with two errors ; 

 wherras, when taken in the manner which the nature of 

 this instrument requires, no more than two errors are 

 charged upon the amount of observations, however often 

 these may have been repeated. 

 To shew more particularly bow this is performed, it 



will be necessary to return to our Figure. There the Circle, 

 different parts are represented, as wtien the index and '^"Y"*"'' 

 horizon glasses are parallel to each other ; in which po- 

 sition, an object will be seen in the telescope through the 

 transparent part of the horizon-glass, in exact contact 

 with its own doubly reflected image ; but by moving the 

 index B towards the telescope, the reflected image will 

 pass along to the left, and being followed by the instru- 

 ment, may be made to coincide with any object seen di- 

 rectly on that hand ; and now the index B would mark 

 the distance between the reflected and direct objects, but 

 it is totally unnecessary, and would even counteract the 

 advantages of the instrument, to pay any attention to this 

 point upon the circle. Instead of this, let the index A 

 be carried forwards until a new point of parallelism is 

 found, by making an object coincide with its own reflec- 

 ted image as at first. This being done, take the angle 

 over again, by moving the index B towards the tele- 

 scope, and following the motion of the reflected image to 

 the left. In this manner p'oceed, by alternate observa- 

 tions for parallelism of the glasses, and angular distance 

 of the objects, until you have made a sufficient set. By 

 this beautiful contrivance, a small instrument is, as far a* 

 dividing may be concerned, rendered at least equal to a 

 large one without it ; but as the telescope, by which the 

 contacts are made, is necessarily of small power, large 

 instruments in this respect seem to have the advantage ; 

 yet, as many observations are required to make a set, the 

 error arising from imperfect contact must be considerably 

 diminished on the well known principle of taking means. 



Professor Mayer proposed, that his new instrument 

 should be 16 inches diameter ; and for the purpose of 

 relieving the arm of the observer from the weight of so 

 heavy an apparatus, recommended that it should be sup- 

 ported by a staff, which should rest upon the ground or j mDrore _ 

 deck. Bird, who probably made the first of these in- me , lt , (, 

 strumenu, improved upon that idea. He made the staff Bird, 

 communicate with a belt, which buckled round the loins 

 of the observer. The staff was double, being composed 

 of two sliding tubes. These moved one within the other, 

 for the purpose of adjusting the height of the circle to 

 the eye of the observer, and by the help of a ball and 

 socket attached to the centre of the instrument, he was 

 enabled to accommodate its plane to the position of the 

 objects. Bird also applied darkening glasses to the re- 

 flecting circle, and gave it every useful appendage which 

 Hadley's quadrant had borne before. In particular it 

 should be mentioned, that the telescope was attached to 

 the index A by two double levers, which acted exactly 

 in the same manner that the single ones do in the com- 

 mon parallel ruler. On these levers the telescope rose 

 and fell, respecting the line of separation between the 

 silvered and unsilvered parts of the horizon-glass. A 

 consequent backward and forward motion necessaiily took 

 place. This is a much better contrivance for regulating 

 the brightness of the two images, than that which 

 was adopted after wards 'for the same purpose by Bor- 

 da. It is a cunous circumstance, that Bird divided hi* 

 circles into 360 instead of 720, as the nature of all 

 reflecting instruments require ; whence it became neces- 

 sary to double the angle read off in a single observation, 

 and in continued ones, to divide the accumulated angle 

 by only half the number of repetitions. 



It may be considered an unfortunate circumstance for 

 the credit of the reflecting circle, that Bird was its ori- 

 ginal maker \ for the correctness of his dividing, deter- 

 mined Admiral Campbell to neglect the repeating prin- 

 ciple altogether, which he deemed a troublesome mode 

 of observation, and to use a single portion of the limb. 

 He then saw, that the arc of the octant ought to be ex- 



