ON AN IMPROVED REFLECTING CTRCT.r. \ \X 



be again effected ; and the nonius will advance the' fajne 

 quantity over the limb. The lame addition will take plsjce, 

 by inverting the ufe of the clamps, and making another ob fer- 

 vation of the diftance. The like alternate procefs may be 

 continued indefinitely ; and the refult given by the inftruinent 

 will, with only one obfervation more, be the fame as that of 

 Borda's method, and double the arch which would be ob- 

 tained by Mayer's circle. 



Mr. Borda's circle is liable, to a very great inconvenience j in Eafy contrivance 

 pra&ice. Each index advances fucceffively over the limb ; ^ft^"^^ 

 and, in order to facilitate the operation of bringing the images before each ob- 

 for the contact within the telefcope, that author advifes -to fervation « 

 make a preparatory memorandum of the pofitions which 1 he 

 indexes will nearly occupy, fo that they may be fet accord-- - 

 ingly, previous to each obfervation. But this method, whiich 

 is always inconvenient, by night becomes almofl impoffib le. . 

 For this reafon, I have joined to the horizon index an audi 

 LL, (Plates XXX. and XXXI.) which is divided, both to 

 the right and left, into degrees and minutes^of the fextant, fo 

 that, when theglaffes are parallel, the centre index coincides 

 with the two firft divifions o, o, and occupies the fpace hjft 

 blank between them. I have further provided two Hiding 

 pieces P, P, which may be adapted to that arch, with a fprin g 

 Sufficient to keep them firm in any fituation. Putting eadi c4" 

 thefe pieces upon the arch, fo that their ends may coincide 

 with the divifions marking the rough distance to be me&fured, 

 no more will be required, than to (ct the centre indox alter- 

 nately againft each piece, before the beginning of the fuccef- 

 iive croffed obfervation. The clamp may then be fattened, 

 and the remainder of the motion produced by the adj lifting 

 {crew ; as, if necelfary, the index will pufli the Aiding piece 

 further, and leave it at the point where the contact was 

 effected *. 



The flying circle facilitates the ufe of any number of no- Other advan- 

 niufes, which may be applied round the whole circumference ; t^incrwfe/ 8 

 but, as the leading principle which I have chiefly had in con- number of no-- 

 templation, is that of obtaining an accurate refult from one nw . "» ' rc " 

 reading, I have only ufed a tingle nonius. Two noniufes, 



* The idea of this fimple contrivance, was fuggefted to me by 

 the ingenious Mr. E.Troughton. .■/... f 



oppofite 



