622 THE LEYDEN JAK. 



which are solid, or at least not open around the screw. From one of 

 these knobs or from a solid ball with a stalk, a suiteible knob is made 

 by filing the screw or stalk into a flat short piece, I*- 5 thick and l cm 

 long, which is done by clamping it horizontally in the jaws of the 

 vice so as to project above the cheek to nearly the middle, taking 

 care to present the * safe ' (uncut) edge of the file used for the purpose 

 to the ball, and thns avoid scratching it. When one side is filed flat 

 it is turned downwards and the other side is then flattened. If a 

 stool-ball with a screw is used, the projecting portions of the worm 

 must also be filed off the sides. The rod to which the knob is to be 

 fixed is then clamped upright in the vice, and a cut is made in it 



FIG. 325. 



with a metal saw. The flat stalk left on the ball is filed until it 

 fits into this cut and is then fixed into it by solder. Fig. 324, .B, 

 shows the end of the rod and the ball before they are fixed together. 



A screw l cm long is cut at the lower end of the metal rod and 

 two small square nuts are prepared for it. Between the two 

 nuts is clamped an elastic strip of brass, 6 or 10 mm wide, with a 

 hole in the middle, and bent as shown in fig. 325. The length 

 of the strip should be such that the distance between the two bent 

 extremities is somewhat greater than the width of the jar for 

 which it is used. The strip is made elastic by hammering ; sheet 

 brass, O mm< 5 thick, is taken and the thickness reduced to about O mm< 3, 

 which gives sufficient elasticity. 



The tinfoil is first attached to the inside. Strips are cut, 4 cm or 

 6 cm wide (for large jars 8 cm or 12 cm ), and long enough to reach 

 from where the coating begins to the bottom of the jar and to leave 

 still about l cm for the bottom itself. A layer of starch-paste being 

 brushed over one of the strips, it is placed inside in the proper posi- 

 tion and fixed by pressure and rubbing with the finger at the top, 



