[Illustration:  Fig. 187.  The Club Pin.]

The Combination Lock.

We found it necessary to close the entrance of our cave with a door fastened with a padlock, so as to keep meddlers out.  The entire school had watched us build the cave house, and, of course, knew just where our entrance lay.  Then, in addition to the outer door, we put in another one, half-way down the dark passageway.  On this Bill rigged up a simple combination lock which would baffle anyone who managed to pick the padlock.  This inner door opened outward.  It was hinged to the floor of the passageway, and swung up against a frame set in the passageway.  At the top was a board whose lower edge lay flush with the edge of the door when it was closed.  For the combination lock we used a couple of spools, each with one head cut off and the central hole plugged up with a stick of wood.  In the door and the top board of the frame, holes were drilled just large enough for the shanks of the spools to fit snugly in them.  Next we made a trip to a hardware store for a file and a couple of large copper washers, about 1-1/4 inches in diameter.  The washers were fastened to the inner ends of the spools after they had been pushed through the hole.  The washer on the door came just to the edge of the door, while the other extended below the door frame and lapped under the door washer.  Then in the edge of the washer on the frame a notch was filed, while in the other washer two notches were filed, so as to leave a tooth which fitted snugly into the notch of the first washer (see Figs. 188, 189).  The door was locked by turning both the washers until the notch and tooth came in line with each other, then pushing the tooth through the notch, and turning the washers so that the frame washer hooked over the door washer.  Then the door could be opened only when the tooth and notch were brought in line.

On the head of each spool we pasted a disk of white cardboard, the edge of which was graduated, as in Fig. 190.  Then we had a secret combination, say 11-19, which meant that when the spools were turned so that the number 11 on the door spool came in line with the number 19 on the frame spool the tooth and notch would be in line, and the door could then be opened.  Of course, this combination was known to the members of the club only, and anyone outside who tried to open the door might have tried for some time without bringing the tooth and notch into line with each other.  Occasionally we changed the combination by loosening the screws which held the washers, and turning them so that the notch and tooth came opposite different numbers on the dials.  This was done so that if anyone should chance to learn our combination he could not make use of it very long.

[Illustration:  Fig. 188.  The Notched Washers.]

[Illustration:  Fig. 189.  Washers Fastened on Spools.]

[Illustration:  Fig. 190.  The Combination Lock.]

CHAPTER XVI.