ON AN IMPROVED REFLECTING CIRCLE. 9 



three representations, in which the fame parts are marked 

 with the fame letters, are fufficient to give an accurate idea of 

 the arrangement of the whole, and make it unneceflary for me 

 to enter now into a minute detail of the mechanifm of the ap«r 

 paratus. I therefore fliall content myfelf with adding here 

 only what concerns the general ufe, and the peculiar proper- 

 ties, of this initrument. 



M is the divided limb of the circle, and N the Flying The circle is Hi- 

 Nonius, to each of which the horizon index may be occa- vided int0 who!c 

 iionally attached, by means of the clamps D, C ; as well as flying nonius 

 the centre index, by means of the other clamps A, B. The has its divifions 

 peculiar property of the initrument being that of giving dou- "* n a \ n c ircle , 

 ble the diftance, I have thought proper to divide the circle 

 into 360 degrees, and not iato 720 according to the nature of 

 the fextant. Thus, after a eroded obfervation, the reading 

 of the Nonius will, without reduction, exhibit the meafure of 

 the fimple diftance. I have likewife extended the nonius 

 round the circumference, fo that, by the coincidence of two 

 divifions, the number of degrees will appear on the limb, 

 and that of the minutes and feconds on the flying nonius. 

 The manner of making the obfervations with this initrument 

 is as follows : 



Adapt the o of the flying nonius to 360° of the limb; and Method of ob- 

 then fatten the two clamps A, B, of the centre index *EE, fc £***' < T amp 

 by which the diviiions will be kept in the fame relative fitua- both to the limb 

 tions. Then, tnrn round the horizon index FF, and make and nonius ( lcC 

 an obfervation of the diftance to the right. The contact mull 360 9 ). Obferve 

 be adjuiled by the fcrew G, the clamp C being fattened, an angle to the 



right by moving 

 the horizon in- 

 fant to the whole circumference, and making it capable thereby of dex ; which 



performing Borda's croffed obfervations, with as many noniufes as t . en c!am P ta 

 r ° . the nonius, 



may be attached to the centre glafs. But, Mr. Troughton's in- Dif en gage the 



ftrument is deprived of the principle invented by Mayer, for ob* center index 



taining at one reading a multiple of the diftance required j which is ' rom tne n °n'«6» 



r • 1 j • • • I 1 /1 r jnd re P eat thc 



the great property or circles, and, in my opinion, the belt means or obfervation to 

 diminishing their errors. the left : the 



* Properly fpeaking, neither the centre nor the horizon indexes flying nonius 

 aft, in this inftrument, as fuch, both of them being deprived of the d/flance. 

 nonius, which is transferred to the flying circle ; but, for the fake 

 of perfpicuity, I continue the ufe of thofe expreflions, in order to 

 diftinguifh the plates or rules which carry the centre and the horizon 

 glaJTes. 



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