al, 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