o it when the indentations were made.

WIRELESS TELEGRAPHY.--Wireless telegraphy depends for its action on what
is called induction. Through this property a current is made of a high
electro-motive force, which means of a high voltage, and this disturbs
the ether with such intensity that the waves are sent out in all
directions to immense distances.

The great discovery has been to find a mechanism sensitive enough to
detect the induction waves. The instrument for this purpose is called a
coherer, in which small particles cohere through the action of the
electric waves, and are caused to fall apart mechanically, during the
electrical impulses.

PRINTING TELEGRAPH.--The printing telegraph requires the synchronous
turning of two wheels. This means that two wheels at opposite ends of a
wire must be made to turn at exactly the same rate of speed. Originally,
this was tried by clock work, but without success commercially, for the
reason that a pendulum does not beat with the same speed at the equator,
as at different latitudes, nor at altitudes; and temperature also
affects the rate. The solution was found by making the two wheels move
by means of a timing fork, which vibrates with the same speed
everywhere, and under all conditions.

ELECTRIC MOTOR.--The direct current electric motor depends for its
action on the principle that likes repel, and unlikes attract. The
commutator so arranges the poles that at the proper points, in the
revolution of the armature, the poles are always presented to each other
in such a way that as they approach each other, they are opposites, and
thus attract, and as they recede from each other they repel. A dynamo is
exactly the same, except that the commutator reverses the operation and
makes the poles alike as they approach each other, and unlike as they
recede.

Steel is simply iron, to which has been added a small per cent of
carbon.

Quinine is efficient in its natural state, but it has been made
infinitely more effectual by the breaking up or changing of the
molecules with acids. Sulphate of quinine is made by the use of
sulphuric acid as a solvent.

EXPLOSIONS.--Explosions depend on oxygen. While this element does not
burn, a certain amount of it must be present to support combustion.
Thus, the most inflammable gas or liquid will not burn or explode unless
oxygenized. Explosives are made by using a sufficient amount, in a
concentrated form, which is added to the fuel, so that when