* Mahogany Wood Putty [139]

Mix venetian red with quite thick arabic muscilage, making it into
a putty, and press this well into the cracks of mahogany before
finishing. The putty should be colored to suit the finish of the
wood, says the Master Painter, by adding such dry color to the gum
as will give the best result.

** How to Make a Thermoelectric Battery [140]
By Arthur E. Joerin

A novel way of producing an electric current by means of hot and
cold water, heat from a match or alcohol

[Illustration: Details of Battery]

lamp, is obtained from a device constructed as shown in the
sketch. Take two hardwood boards, marble, or slate plates, about 8
or 10 in. long, place them together, as in Fig. 1, and mark and
drill about 500 holes. These two pieces should be separated about
8 in. and fastened with boards across the ends, as shown in Fig.
2.

Take soft copper wire, not smaller than No. 18 gauge, and cut in
lengths to pass through the holes in the two boards, leaving
sufficient end to make a tie. It will require about 70 ft. of wire
to fill one-half the number of holes. Also, cut the same number of
lengths from the same gauge galvanized-iron wire to fill the
remaining holes. The wires are put through the holes in the boards
alternately, that is: begin with copper, the next hole with iron,
the next copper, the next iron, and so on, twisting the ends
together as shown in Fig. 3. The connections, when complete,
should be copper for the first and iron for the last wire.

When the whole apparatus is thus strung, the connections, which
must be twisted, can be soldered. Connect one copper wire to the
bell and the other terminal, which must be an iron wire, to the
other post of the bell. The apparatus is then short-circuited, yet
there is no current in the instrument until a lighted match, or,
better still, the flame of an alcohol lamp is placed at one end
only.

Best results are obtained by putting ice or cold water on one side
and a flame on the other. The experimenter may also place the
whole apparatus under sink faucets with the hot water turned on at
one terminal and the cold water at the other. The greater the
difference of temperature in the two terminals, the more current
will be obtained.

Very interesting experiments may thus be performed, and these may
lead to the solving of the great thermoelectric problem.

** How to Make a Hygrometer [140]

Mount a wire on a board which is used for a ba