The core is made by pushing a small nail through a piece of cork.  It should be made so that it will rise slowly when placed under water.  Some filing may be necessary to get the weight just right, but it should be remembered that the buoyancy of the core can be adjusted after the parts are assembled, by pressing the cork in the bottom of the test tube.  This causes compression in the water so that some is forced into the upper cork, reducing its displacement and causing it to sink.  The lower cork is then slowly withdrawn, by twisting, until the core slowly rises.

[Illustration:  Galvanoscope]

The instrument will then be adjusted ready for use.  Connect the binding posts to a single cell of battery—­any kind will do, as a slight current will answer.  On completing

[Illustration:  Interior View]

the circuit the core will descend; or put in a switch or push button on one of the battery wires.  If the button be concealed where the operator can reach it, the core will obey his command to rise or fall, according to his control of the current.  This is a mysterious looking instrument, the core being moved without visible connection to any other part.

** Lubricating Sheet Metal [69]

To lubricate sheet metal mix 1 qt. whale oil, 1 lb. white lead, 1 pt. water and 3 oz. finest graphite.  Apply with a brush before the metal enters the dies.

** An Optical Top [69]

One of the latest optical delusions, and one not easy to explain, is Benham’s color top.  Cut out the black and white disk shown in the figure, and paste on a piece of stiff cardboard.  Trim the edges of the cardboard to match the shape of the disk, and make a pinhole in the center.  Cut the pin in half and push it through from the under side until the head of the pin touches the cardboard.  Spin slowly in a strong light and some of the lines will appear colored.  The colors appear different to different people, and are changed by reversing the rotation.

[Illustration:  An Optical Top]

** Card Trick with a Tapered Deck [70]

Another simple trick to perform but one not easily detected, is executed by using a tapered deck of cards as shown in Fig. 1.  A cheap deck of cards is evened up square, fastened in a vise and planed along the edge in such a manner that all the pack will be tapered about 1/16 in.  This taper is exaggerated in the illustration which shows

[Illustration:  Cards from a Tapered Deck]

one card that has been turned end for end.

It is evident that any card reversed in this way can be easily separated from the other cards in the pack, which makes it possible to perform the following trick:  The performer spreads the cards out, fan-like, and asks an observer to withdraw a card, which is then replaced in any part of the pack.  After thoroughly shuffling the cards the performer then holds the deck in both hands behind his back and pronouncing a few