f(yr Measuring the 'Expansions of Solids, %c. 197 



quantities in astronomical and geodetical operations. For 

 this purpose the scale is constructed of two rules of brass, 

 accurately joined together at a right angle by their edges, and 

 fitting square upon two sides of the black-lead bar, and of 

 about half its length. At one end of this double rule a small, 

 plate of brass projects at a right angle, which plate, when the 

 two sides of the former are applied to the two sides of the re- 

 gister, is brought down upon the shoulder formed by the notch 

 cut away at its upper end, and the whole may be thus firmly 

 adjusted to the black-lead bar by three planes of contact. 



On the outside of this frame another brass rule is firmly 

 screwed down, which projecting beyond it, and bending a little 

 so as to bring its end opposite to the cavity in the black-lead 

 bar when applied to it, supports a moveable arm exactly 5^ 

 inches long, turning at its fixed extremity upon a centre, and 

 at its other carrying an arc of a circle, accurately divided into 

 degrees and thirds of a degree, whose radius is exactly 5 inches. 

 At the centre of this circle upon the arm, and of course at the 

 distance of half an inch from the centre of motion, another 

 lighter arm is made to turn, one end of which, being the exact 

 radius of the circular arc, carries a nonius with it, which moves 

 upon the face of the arc and subdivides the former graduation 

 into minutes. The other end crosses the centre ; and at the 

 exact distance of one-tenth of the radius, or the distance be- 

 tween the two centres of motion, terminates in an obtuse steel 

 point turned inwards at a right angle. These graduations 

 and distances are laid down with the greatest precision by 

 Mr. Troughton's dividing engine. This part of the apparatus 

 may be regarded as a pair of proportional compasses attached 

 to the end of tlie brass rule and frame, whose longer legs 

 carrying the arc and nonius are to its shorter as ten to one ; 

 and the opening of the latter being regarded as a chord of a 

 small circle, is magnified in the same proportion by the for- 

 mer, and measured upon the scale. A small steel spring let 

 into the larger arm is made to press upon the smaller, so as 

 to adjust the nonius to the commencement of the graduation ; 

 and when forced back it tends to restore it to its original 

 position. 



The annexed figures, in which all the parts are drawn of 

 their real dimensions*, will assist the comprehension of the 

 preceding description. Plate II. fig. 1. represents the scale. 

 A A is the principal brass rule, upon the under side of which 

 the frame a a a a a a' is adjusted by the screws b b, and which 



* In our plate the pyrometer is rciircsentcd about one tliird less tluin 

 its real diiiieiisiunii. — lion. 



supports 



