230 Description of an improved Reflecting Circle, 



of the instrument in all its positions ; I have therefore con- 

 trivLcl to put what is called in our business, a ntale and fe- 

 male centre or axis, upon a simple but accurate method. 



Permit me to make a fi'w observations on circular instru- 

 ments in general. 1 believe it will be universally allowed, 

 th^t it is easier to make a circle nearer to truth, witli respect 

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

 a circle so. 



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

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

 not turned on its own axis as a circle is ; therefore 1 have 

 no doubt, but that the best sextant usually made, is very 

 short of the horizontal truth of a sixth part of a circle ; and 

 if we were to suppose a circle made of six of the usual sex- 

 tants, it would be a very untrue circle with respect to its 

 horizontal plane. 



It has, therefore, been a genera! desideratum, that a cir- 

 cular instrument of reflection should be introduced, of 

 simple construction, easy to adjust, and convenient for use. 

 I have been induced to make several circular instruments 

 of reflection in various ways, but none upon so simple a 

 construction, or so cheap, as the present, nor so well cal- 

 culated to prove any untruth, as my improvement upon 

 Borda's; and I believe it will now be generally adopted for 



use. 



There have been great numbers of Borda's circles made; 

 I myself assiwled about twenty-five years ago to make many, 

 also since I have been in business for the last twelve years 

 on my own account, but I never found any of them to give 

 satisfaction till I invented the present improvement. 



Captain M'Lennan, who traded to South America, had 

 one of Borda's Circles made, similar to that described in 

 Dr. iVlackay's Longitude, but could not use it till altered 

 by me last April. 



The glasses in my instrument are moveable to any quar- 

 ter that a person may wish to use it in; and by taking the 

 same angle with each quarter, it aff'ords an opportunity of 

 proving the correclness of the instrument, which circum- 

 stance I hope justilies me in saying, that it is the only in- 

 strument of reflection that I know, so ivell calculated to 

 prove itself. I beg pardon for being so tedious ; I assure 

 you that I can make the instrument better than I can write 

 or talk about it. 



I have the honour to be, sir, 

 Your humble servant, 



Blewitts Buildinofs, Jan. 16, 1811. JamES AlLAN. 



To C. Taylor, M.D. Sec. De- 



