ON A»: IMPROVED REFLECTING CIRCLE. 



From 1 to 4 



TABLE II. 



From 5 to 9 



From 10 to 16 



Advantages of 

 large instru- 

 ments in dimi- 

 nishing the er- 

 rors of divifion 

 and eccentricity. 



Invention of 

 Mayer for dimi- 

 nishing the fame 

 errors at p!ea- 

 furc in fmall 

 inftruments"ty 

 reflection. 



II. 



On an imp roved Reflecting Circle. By Joseph deMendoza 

 Rios, Eft. F.R. S. {Phil. TranJ\ 1801.) 



-tt N practical aftronomy large inftruments are ufeful, not only 

 to enable the obferver to read the angles to a fmall fraction of 

 a degree, but likevvife to diminifh, in the conftruction, the 

 inaccuracies which proceed both from the errors of the divi- 

 fions and the eccentricity of the index. Frames of confide- 

 rable dimenfions admit alfo the application of telefcopes with 

 great magnifying powers, which is a circumflance of the ut- 

 inoft importance in celeftial obfervations. As the reflecting 

 inftruments employed at fea are fupported by the hand, their 

 weight and fcale are limited within a narrow compafs ; and it 

 leemed very difficult to obviate, by any expedient, the incon- 

 veniences arifing from the fmallnefs of their fize, while it was 

 impoflible to increafe it. The celebrated Tobias Mayer con- 

 trived, however, a method to determine, at one reading, in- 

 uead of the fimple angle obferved, a multiple of the fame 

 angle ; and, by this means, the inurnment became, in prac- 

 tice, capable of any degree of accuracy, as far as regards the 

 above mentioned errors. His invention is eifentially different 

 from the mere repetition of the obfervations j and my object 

 requires that I fhould explain the principle upon which it is 

 founded. 



Mr. 



