01' THE UNITED STATES. 317 



The verniers arc all ot double length, having on each side 

 of the usual vernier one liall'length oi" vernier over. In- which 

 means two readings are obtained. A mean of these read- 

 ings can betaken, in case of any diflerence. 



The whole framing of the instruments is in general strong- 

 er in every respect than tlicy were made l)efoie, and parti- 

 cular attention is paid to the staltility of all the adjusinients, 

 as may be observed from a mere comparison of this figure 

 with those in tiie Kncyclopedia. 



'I'lie breadth oi'thc ring to which the radii of the instru- 

 ment arc fastened being al)out one incii. the distance at which 

 tiie verniers W, X, ^'c. of the back alhidadc must be sup- 

 ported by it would twist this alhidadc by their weight, whicli 

 would occasion them to read did'orcntly indilfcieiu positions 

 of the circle, as they would in all cases sink. This is avoid- 

 ed by adding on the side opposite to the vernier the nobs 

 II. (I, forming a counterpoise to the verniers, and effecting 

 thereby tlicir verticality in all positions. 



To give more stability to the vertical or any inclined po- 

 sition of the circle, there are two semicircles k, k fastened 

 to the horizontal axis q round which the circle is moved, 

 close to the uprights supporting this axis at both ends, to 

 which they are clamped I)y two screws /, /, instead of only 

 one as in the former construction. There is also a small ball 

 s upon the socket of the axis of the circle, between the coun- 

 terpoise and the clamping arrangement ii of the circle, ad- 

 justable by a sliding |)iece in the small upright /, which re- 

 ceives one of the axes of this level. The level swings on 

 an axis, to serve in both opposite positions in which the cir- 

 cle may be placed by revolving upon the semicircles. 



It appears to me, that in all instruments the alhidades 

 should l)e clamped on a se|)aratc arm not bearing any read- 

 ing, (when these arc not upon a full circle.) Iiecause the 

 clamp is very apt to affect the reading to which it is adapt- 

 ed. For this reason, tlie clamp and tangent screw of the 

 hack alhidade is put to the separate arm v. perpendicular to 



VOI-. II. — T 2 



