444 Major-General . Roy’s Account of the 
The fixed apparatus confifts of a cork about three inches in 
length, made of the very befb material, and fo nicely fitted to 
the bore as juft to admit of being forced into without burfting 
it. In the middle of the cork a cylindrical brafs tube is placed, 
whofe fides are thin, the inward end thick, and the outward 
end open. It receives a fteel pin, whofe inward end being 
formed into a fcrew, is thereby fixed into the thick metal of 
the tube. The fteel pin carries outwardly a button and neck 
of bell-metal. The neck fits fo very clofely the open end of 
the brafs tube as to prevent any fhake there ; • at the fame time 
that the infide of the button applies very juftly to the ground 
end of the glafs tube, to which the outward furface (being 
a true plane) is exactly parallel. 
The moveable apparatus confifts, like the other, of a cork 
and brafs tube of the fame length. Before the infertion of this 
cork, an oblong piece feVen -tenths ®f an inch long, and two- 
tenths broad, was cut from it, in that part of its cylinder an- 
fwering to the upper part of the outward end of the glafs tube, 
on the inward furface of which, about half an inch from the 
end, a fine line had been previoufly cut by a diamond point. 
The brafs tube in this cork contains within it a loofe fteel 
* » 
worm, or helical fpring, fomethmg lefs than the interior dia- 
meter of the tube. Along the cavity formed by the fpiral, 
there pafles a fteel pin, like that in the fixed end ; but it i-s 
longer, and has no ferew at the inward end, that being nicely 
gfound, fo as to fit a circular hole in the inward end of the 
brafs tube, while a triangular bell-metal neck fits one of that 
figure in the outward end. Thus the pin moves freely back- 
wards or forwards without any fhake, and preftes upon the 
fteel fpring, by means of a circular brafs collar, placed for the 
purpofe, at the inward end of the neck ; while the outward 
■ 5 end 
