Fig. 53. The Umbrella Rib Crossbow. and a nail was laid in the channel with its head against the bowstring. Then, on pulling the trigger, the bowstring was lifted out of the notch, and sent the nail off sailing. The long-grooved barrel insured a very good aim.
Megaphone.
Fig. 54. The Megaphone.
Another device we made in preparation for the expedition was a megaphone. A sheet of light cardboard 30 inches square was procured. At the center of one edge a pin was stuck into the cardboard, then a piece of stout thread was looped over the pin and the two ends were knotted together just
Fig. 55. Layout of the Megaphone. 5 inches from the pin. Another knot was also made 29 inches from the pin. Now, with a pencil hooked into the loop, and resting first against the inner knot and then against the outer one, two arcs were drawn on the paper, one of 5-inch radius and the other of 29-inch radius. A line was now drawn from the pin to the point where the longer arc met the right hand edge of the paper, and a dotted line was drawn from the pin to a point
