Meafurement of a Baft on Houn flow- Heath. 465 
the firft of thefe, two ftrong pieces of brafs, fixed to the cheeks, 
and notched underneath, embrace the ends of a brafs cylin- 
drical bar fattened to the bottom. At the lafl, the cheeks of 
the Aide reft on a roller. Whence it follows, that the copper and 
Aide remain immoveable with regard to each other at A ; but 
from thence, towards either end, they have full liberty to 
change place ; that is to fay, to expand by heat, or contract by 
cold, in any proportion their different natures may require. 
The left-hand end of the Hide is (hut up by a ftrong perpen- 
dicular piece of brafs, conne&ed with the two fide rings which 
fupport the obje& lens of the fixed microfcope, whofe center 
correfponds accurately with its inward face. This piece being 
firmly ferewed to the cheeks of the Aide, and counter-arched 
outwardly, forms a ftrong butt for the fixed end of the ex- 
panding rod (fuppofed here to be the fteel bar) to aft againflv 
Within the right-hand end of the long Aide, refts a fliort one 
of about 14! inches in length, whofe cheeks are if inch deep. 
Its outward end, at C, refts on the cylindrical furfaCe of the laft 
brace of the long Aide, fitted purpofely to receive it ; while a 
narrow longitudinal bar fixed in its inward end, at Du, in the 
feftion, moves freely in the notch of a bridge b , framed for it 
in the long Aide. The outward end of this Aiort Aide is (hut 
Up in a fimilar manner with the oppolite end of the long one. 
This end-piece is aifo connected with the two fide rings 
which fupport the tube containing the object lens of the mi- 
crometer microfcope, whofe center is perpendicularly over its 
inward face, and being fortified outwardly by an edge bar, it 
forms a butt for the expanding end of the rod that is in experi- 
ment to pufii againft. By attending to the' plate it will be 
perceived, that to this end of the boiler a brafs tube (R) i§ 
fixed, which contains within it a brafs rod, furrounded by a 
p n n ~ helical 
