FOR MEASURING THE EXPANSION OF SOLIDS. 
263 
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 register, 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 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 between 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 the 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 former, 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 dimen- 
sions, will assist the comprehension of the preceding description. Plate X. 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 supports 
upon its bent extremity c, the arm B moving upon the centre d, and termi- 
nating in the arc of the circle e e. 
2 M 
MDCCCXXX. 
