The upper end of the connecting rod should be turned over to form a finger
rest, or be sharpened off to take a knob. The last operation is the fitting
of the spring to close the shutter. A spiral spring attached at one end of
the crank on the lower spindle and at the other to a nail projecting from
the side of the front is the most convenient arrangement. If you have not
got a spiral spring, you can easily make a. fairly efficient substitute out
of hard brass wire wound a few times round a large wire nail.
An alternative method of springing is to add an arm, a, to the connecting
rod, as shown by dotted lines in Fig. 189, and to use the projection for
engaging a spring, made by winding hard brass wire a few times round a
nail. A screw passed through the coil holds it to the front.
The tension of the spring must be just sufficient to close the shutter
smartly and prevent it rebounding far enough to pass any light.
XXXIX. A MINIATURE GASWORKS.
The most primitive method of making coal gas on a small scale is to fill a
tin--which must have folded, not soldered, joints--with small coal, punch a
hole in the bottom, and place it lid downwards in the fire.
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