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XXXVI. Description of an Improvement on La Eorda's 

 Reflect tug Circle. By Mr. J. Allan *. 



Sir, 1 BEG leave to inform you, that on Thursday last, I 

 left at the Society's house, a mathematical instrument, 

 adapted for the use of mariners, which I wish to submit to 

 the Society's attention. It is a Reflecting Circle, com- 

 monly called la Borda's Circle, for the purpose of taking 

 altitudes and distances at sea; and which I have s,Teatly 

 ' improved lately, by fixing the shade glasses different to 

 wiiat had heretofore been done, vvith some otlier improve- 

 ments as a Reflecting Circle. The late Dr. Mackay, in a 

 publication of his called Mackay's Longitude, has a plate 

 of La Borda'8 original instrument, but the shade glasses are 

 so fixed, as to render the instrument useless, and which he 

 was convinced of on my pointing out to him the fault ; 

 he said he would alter Ins plate to my method, and that he 

 would state it as my improvement; but his death soon af- 

 terw^anls prevented ii. 1 am aware the Society do not con- 

 fer their rewards without advantageous qualities to -merit 

 their sanction. I respectfully say"^ that 1 consider my in- 

 strument to have merit, both in ceconomy, and in the oreat 

 improvement made on the plan of the Reflecting Circle 

 first invented. I shall be happv to point out; this to the 

 Society, and have the honour to be, 

 Sir, 



Your humble servant, 



Blewj'tL's Buildings, Fettoi- Lane, Dec. 24, 1810. JamES AlLAN, 



To C. Tmjlor, M.D. Sec. 



Sir,— Agreeably to the intimations of the Committee 

 on 'fhjrfday evening last, 1 beg leave to explain to the So- 

 ciety, the properties of i.iy improved Reflecting Circle ; and 

 which, with a Theodolite attached to it, would be useful 

 b!)th to the m.iriner and surveyor. 



The Committee inquired wliat sort of centre or axis the 

 instrument had. I beg leave to stale, it is an improved one 

 of mine. The former way of centering this instrument 

 was only by a sirf^le pin, wiiich both indexes acted upon ; 

 but the pin had so little bearing in the index, that ii was 

 not sufficient to keep the index-glass upright to the plane 



• From Trmisaclions of Ihe Sociely for Hie Encnuragement if /iris, Manufac- 



lurts, auid Commerce, for 1811 Tiie silver medal of ihe Sociciv and 



twenty guineas were voted t(j Mr. Alhm, for this improvement. 6:ie (;f 

 the»e i«struiiierirs is prc-eivcd i.i liic- Society's Repotitorv. 



Vol. 39. No. 168. April 1812. R of 



