lU 



IMPROVED REFLECTING CIRCLE. 



Reflecting cir- 

 cles made in 

 various ways. 



Eorrla's unsa- 

 tisfactory. 



The improved 

 circle well a- 

 dapted for 

 proving its 

 correctness. 



Testimonials 

 in its favour. 



the horizontal truth of a sixth part of a circle ; and if we were 

 to suppose a circle made of six of the usual sextants, it would 

 be a very untrue circle with respect to its horizontal plane. 



It has, therefore, been a general desideratum, that a 

 circular 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 calculated 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 assisted 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 satis- 

 faction 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. 

 Mackay's Longitude, but could not use it till altered by me 

 last April. 



The glasses in my instrument are movable to any quarter 

 that a person may wish to use it in j and by taking the same 

 angle with each quarter, it affords an opportunity of proving 

 the correctness of the instrument, which circumstance I hope 

 justifies rne in saying, that it is the only instrument of reflec- 

 tion that I know, so well calculated to prove itself. I beg 

 pardon for being so tedious j 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, 



JAMES ALLAN. 

 BkwitCs Buildings, Jan. l6, 1811. 



A certificate was produced from Captain H. C. Coxen, R.N. 

 dated February 5th, 1811, stating his opinion, that the dark 

 screens which are fixed lo Mr. Allan's reflecting circle, so as to 

 act in the manner they do in a sextant, are improvements on 

 the reflecting circle of Mr. Borda, which are not so fixed. 



That it must be evident, even to the least experienced 

 mariner, that there are frequent occasions, in taking the altitude 

 of the sun, to change the screens alternately, in the bhortest 



possible 



