Description of an improved Refecting Circle. Sj I 



Description of the Drawing of Mr. James Allan's Im- 

 provement on tkeRefecUng Circle of Borda, PI. V. 



The Reflecting Circle, first invented by Tobias Mayer of 

 Gottinfjen, and'^arierwards improved by the Chevalier La 

 Bordaj^of Pari^, is an instrument, which in its principle 

 admits of such a degree of accuracy, as to be ot the most 

 important service to navigators; but it has hitherto been 

 constructed in such a manner, that the inconveniences at- 

 tending the use of "it have prevented its general adoplioa 

 among seamen ; any contrivances, theretore, lending to 

 diminish these inconveniences, were deserving ot the So- 

 ciety's notice. The construction of Borda's c-trcle as they 

 have hitherto been made, is minutely detailed in Dr. Rees's 

 New Cyclopaedia, article. Circle ; and the mode of using 

 it is there explained; it will be therefore unnecessary to 

 describe anv thing more of the circle delineated in PI. \, 

 than is essential to the elucidation of the improvements 

 made by Mr. Allan. 



The first of these is in the mode of applying the dark 

 glasses, which are fixed on joints, so as to turn back out ot 

 the wav, in the same manner as in the sextant; in the old 

 instrument these glasses were fitted into sockets provided 

 with tenants on the indexes, and fastened by a milled head 

 screw, which took much time to change them ; the second 

 is the addition of double verniers to the index, carrying the 

 telescope and horizon glass ; these read upon opposite sides 

 of the circle, and if a difference is observed between these 

 readings bv taking I he mean of them, the error aruing from 

 any ex-centrici:y ihe index may have, will be corrected: and 

 tlie third consists in fixing the index glass upon an axis, ac- 

 curately titled into the cenire of the circle; by this means it 

 is assured that the index glass in turning round, shall always 

 be exactly perpendicular to the plane of the circle ; in the 

 old method, when the index-bar was merely fitted on a pin 

 fixed in the centre of the circle, it was impossible to make 

 the circle so perfectly flat, or keep the index so accurately 

 in contact wiih it, as by having an axis. To explain those 

 in^provements more perfectly, the reader is referred to plate 

 V, which contains a perspective view of the instrument ; 

 A, is the circle with six arms; B, is ihe index cairying ihe 

 telescope C, and the horizon-glass D, with the U\o clusters 

 of dark glasses E and F; at the opposite ends of this index 

 are the two verniers a and /;, the former has the clamp screw 

 and slow movemtnt attached to it, consisting of a screw c, 

 which lixed the index to the circle j and d is the tangent 

 K 2 screw. 



