OF THE UNITED STATES. 879 



must be considered as iiulispcnsablc. Formerly the artists 

 ill England had for this purpose a still better metal, — the 

 tutenague, imported from China, but now prohibited. The 

 greater hardness of this metal was particularly favourable for 

 the axes of transits, b^c. which, by their frequent revolution 

 upon the same parts, without going completely round, are 

 very apt to become partially worn, and which catuiot be 

 made of steel, on account of its rusting. All the nobs and 

 sockets in which the tangent screws move should be of good 

 cannon metal ; for tiicse screws, being frequently used, will 

 soon lose their easy motion, if they turn in brass, in conse- 

 quence of their close contact and equality of texture ; and 

 after this, they acquire an inefficient or lost motion, to the 

 great annoyance of the observer. To avoid this in all cases, 

 1 think it would be easy to adapt a spiral spring of steel wire 

 around the screw between the two nobs in which the screw 

 works, pressing it always to the same side of the path. 



In the general construction of the instrument, attention 

 must be paid to give to the lower parts such strengtli and 

 weight witii respect to the upper, as may not allow of any 

 spring. This is the great difficulty which requires tire obser- 

 ver's particular care in the repeating circle with two tele- 

 scopes, where the main weight of tlie instrument is support- 

 ed in equilil)rio upon a small axis, so that the vertical obser- 

 vation must in some measure be made without touching the 

 instrument. 



The verniers which present inclined planes to the divisions 

 must toucli them in order to show accurately the coincidence 

 and avoid parallax. This renders it necessary to give some 

 spring to the arms which bear them, and care must therefore 

 be taken, that they do not rub the divisions, and of course do 

 not drag. Mr. Rtichenljach in Munich avoids this by divid- 

 ing the verniers upon a complete circle, which presents it- 

 self inside to the divisions in the same plane. lUit this re- 

 quires very nice work, and as the circle is light, some rare 

 to prevent its warpino;. which may l)c occasioned by a retuiii 

 of some hammered i)arts of it to their former irregular shape. 



