follow directions carefully and then fail to get good results.  The usual trouble is not with the battery itself, but with the circuit.  A gravity battery is suitable only for a circuit which is normally closed.  It is therefore undesirable for electric bells, induction coils and all other open-circuit apparatus.  The circuit should also have a high resistance.  This makes it impractical for running fan motors, as the motor would have to be wound with fine wire and it would then require a large number of batteries to give a sufficiently high voltage.

To set up a gravity battery:  Use about 3-1/2 lb. of blue stone, or enough to cover the copper element 1 in.  Pour in water sufficient to cover the zinc 1/2 in.  Short-circuit for three hours, and the battery is ready for use.  If desired for use immediately, do not short-circuit, but add 5 or 6 oz. of zinc sulphate.

Keep the dividing line between the blue and white liquids about 1/2 in. below the bottom of the zinc.  If too low, siphon off some of the white liquid and add the same amount of water, but do not agitate or mix the two solutions.  This type of battery will give about 0.9 of a volt, and should be used on a circuit of about 100 milli-amperes.

** A Skidoo-Skidee Trick [116]

In a recent issue or Popular Mechanics an article on “The Turning Card Puzzle” was described and illustrated.  Outside of the scientific side involved, herein I describe a much better trick.  About the time when the expression “skidoo” first began to be used I Invented the following trick and

[Illustration:  How to Cut the Notches]

called it “Skidoo” and “Skidee,” which created much merriment.  Unless the trick is thoroughly understood, for some it will turn one way, for others the opposite way, while for others it will not revolve at all.  One person whom I now recall became red in the face by shouting skidoo and skidee at it, but the thing would not move at all, and he finally from vexation threw the trick into the fire and a new one had to be made.  Very few can make it turn both ways at will, and therein is the trick.

Take a piece of hardwood 3/8 in. square and about 9 in. long.  On one of the edges cut a series of notches as indicated in Fig. 1.  Then slightly taper the end marked B until it is nicely rounded as shown in Fig. 2.  Next make an arm of a two-arm windmill such as boys make.  Make a hole through the center or this one arm.  Enlarge the hole slightly, enough to allow a common pin to hold the arm to the end B and not interfere with the revolving arm.  Two or three of these arms may have to be made before one is secured that is of the exact proportions to catch the vibrations right.