IMPE0VE19 KEFLECTING CIRCLE. US 



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

 of La Borda's original instrument, but the shade glasses are so defect in La 

 fixed, as to render the instrument useless, and which he was Borda's. 

 convinced of, on my pointing out to him the fault. He said he 

 would alter his plate to my method, and that he would state it 

 as my improvement ; but his death soon afterwards prevented 

 it. I am aware the Society do not confer their rewards with- 

 out advantageous qualities to merit their sanction. I respect- 

 fully say, that I consider my instrument to have merit, both in 

 economy, and in the great improvement made on the plan of 

 the reflecting circle first invented, I shall be happy to point 

 out this to the Society, and have the honour to be. 

 Sir, your humble servant, 



JAMES ALLAN. 



BlewitCs Buildings, Fetter Lane, 

 Dec.24tli, 1810. 

 SIR, 



Agieeably to the intimations of the committee on Thursday Useful both to 

 evening last, I beg leave to explain to the Society the proper- mariners and 

 ties of my improved reflecting circle j and which, with a theo- 

 dolite attached to it, would be useful both to the mariner and 

 surveyor. 



The committee inquired what sort of centre or axis the j^^^je ^f cen- 

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

 of mine. The former way of centring this instrument was 

 only by a single pin, which both indexes acted upon j but the 

 pin had so little bearing in the index, that it was not sufficient 

 to keep the index-glass upright to the plane of the instrument 

 in all its positions j I have therefore contrived to put what is 

 called in our business a male and female centre or axis, upon 

 a simple but accurate method. 



Permit me to make a few observations on circular instru- Horizontal 



ments in general. I believe it will be universally allowed, P^?"f °^ ? 



° ■' whole circle, 



that it is easier to make a circle nearer to truth, with respect to 



its horizontal plane, than it is to miake a separate part of a • 



circle so, 



A sextant is only the sixth part of a circle, and is got flat ^ade truer 

 by means of a plane, as near as the maker can get it, but is than of a part, 

 not turned on its own axis as a circle is : therefore I have no 

 doubt, but that the best sextant usually made is very shout of 



Vol. XXXIII, No. 152, October, 1812, I th« 



