** To Renew Old Dry Batteries [401]

Remove the paper that covers the cell and knock several good-sized holes in the zinc shell.  Place the battery in a glass jar, fill it two-thirds full of strong sal ammoniac (or salt) solution and connect the terminals to whatever apparatus the current is to be used for.  A few drops of sulphuric acid quickens and improves the action.  The output of the cell will be nearly as great as when the battery was first bought.  —­Contributed by C. W. Arbitt, Austin, Texas.

** Blue Dye[401]

Prussian blue and Chinese blue are both the same chemically but they do not cut or look the same.

** Acetylene lamp [401]

When an acetylene lamp is in good order it will light up slowly with a hissing noise followed by a pure white flame.  Should the lamp light up quickly with a yellowish flame, it is a sign of a leak somewhere.

** Another Electric Motor [401]

This form of electric motor is used largely in England in the form of an indicator.  It is very easily made and

[Illustration:  Electric Motor]

if you have an old electro-magnet will cost practically nothing.

A large soft-iron wheel is mounted on an axle with a pulley-wheel on one end and a circuit breaker on the other end.  The teeth on the circuit-breaker must be the same number as on the soft-iron wheel.

The electro-magnet is mounted so that its core is level with the axle and in a line with the wheel.  One wire from it is attached to one binding screw and the other end is grounded to the iron frame that supports it.  This frame is connected to the frame supporting the wheel.  A small brush presses on the circuit-breaker and is connected to the other binding screw.

In the diagram A represents the iron wheel; B, the brush; C, the circuit breaker; D, the magnet.  The wire connecting the two frames is shown by a dotted line.

To start the motor, attach your battery to the screws and turn the wheel a little.  The magnet attracts one of’ the teeth on the wheel, but as soon as it is parallel with the core of the magnet the circuit is broken and the momentum of the wheel brings another tooth to be attracted.

To reverse the motor reverse the connections and start the wheel the other way.  Be sure that the frames are screwed down well or the motor will run jerkily and destroy the connections.  —­Contributed by F. Crawford Curry, Brockville, Ontario.

** How to Make a Propelling Vehicle [402]

[Illustration:  Fig. 1]

Any boy, with a little knack and a few odd tools, can rig up various contrivances which will be a source of pleasure to himself and oftentimes can be sold, to less ingenious boys, for a snug little sum.  Any tool a boy can obtain is apt to be of use to him, chisel, bit, jack-knife or hammer.