A very economical alloy is made by melting up all the old type-metal, babbitt, battery zincs, white metal and other scrap available, and adding a little antimony if the metal shrinks too much in cooling.  If a good furnace is available, aluminum can be melted without any difficulty, although this metal melts at a higher temperature than any of the metals previously mentioned.

In casting zincs for batteries a separate crucible, used only for zinc, is very desirable, as the presence of a very small amount of lead or other impurity will cause the batteries to polarize.  A very good way to make the binding posts is to remove the binding posts from worn-out dry batteries and place them in the molds in such a way that the melted zinc will flow around them.

The time required for a casting to solidify varies with the size and shape of the casting, but unless the pattern is a very large one about five minutes will be ample time for it to set.  The casting is then dumped out of the mold and the sand brushed off.  The gate can be removed with either a cold chisel or a hacksaw, and the casting is then ready for finishing.

** Battery Switch [99]

In cases where batteries are used in series and it is desirable to change the strength and direction of the current frequently, the following device will be found most convenient.  In my own case I used four batteries, but any reasonable number may be used.  Referring to the figure, it will be seen that by moving the switch A toward the left the current can be reduced from four batteries to none, and then by moving the switch B toward the right the current can be turned on in the opposite

  [Illustration:  Battey Switch]

direction to the desired strength.  In the various positions of these two switches the current from each individual cell, or from any adjacent pair of cells, may be used in either direction.  —­Contributed by Harold S. Morton, Minneapolis.

** An Optical Illusion [99]

The engraving shows a perfectly straight boxwood rule laid over a number of turned brass rings of various sizes.  Although the effect in the illustration

[Illustration:  An Optical Illusion]

is less pronounced than it was in reality, it will be noticed that the rule appears to be bent, but sighting along the rule from one end will show that it is perfectly straight.

The brass rings also appear distorted.  The portions on one side of the rule do not appear to be a continuation of those on the other, but that they really are can be proved by sighting in the same manner as before.  —­Contributed by Draughtsman, Chicago.

** New Method of Lifting a Table [99]

To perform this feat effectively the little device illustrated will be required.  To make it take a sheet-iron band, A, 3/4 in. wide and attach a strap to fasten on the forearm between the wrist and elbow.  Put a sharp needle point, B, through the sheet-iron so that it extends 3/4 in. outward.  Make one of these pieces for each arm.  In lifting the table first show the hands unprepared to the audience and also a tight table, removing the cover to show that the surface of the table is not prepared in any way.  Then replace the table,