^44 Major-General Roy's Account oj the 



The fixed apparatus confifts of a cork about three inches ia 

 length, made of the very bell: 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,, 

 whofc fides are thin> the inward end thick, .and the outward 

 end open. It receives a fteel pin, whole 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 lb very clolely the open end of 

 the brafs tube as to prevent any Ihake 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 exadly 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 of 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 previoully cut by a diamond point. 

 The brafs tube in this cork contains within it a loofe fleel 

 worm, or helical fpring, lomething lefs than the interior dia- 

 meter of the tube. Along the cavity formed by the fpiral, 

 there paffes a fteel pin, like that in the fixed end ; but it i-s 

 longer, and has no fcrew at the inward end, that being nicely 

 ground, fo as to fit a circular hole in the inward end of the 

 brafs tube, while a triangular bell-metal neck fits one ot that 

 figure in the outward end. Thus the pin moves freely back- 

 wards or forwards without any fhake, and prefles upon the 

 fleel fpring, by means of a circular brafs collar, placed for the 

 purpofe, at the inward end of the neck ; while the outward 

 c end 



