Of course, the connections need only be arranged when the device is wanted.  Shops and offices can be guarded by making the current show a red light from a lamp hung in front of the premises, so that the night watchman can see it on his beat.  This can readily be done by adjusting an electromagnet to drop a screen of red glass before the flame of the lamp.  Safes and showcases forcibly opened can be made to signal the fact, and recently in the United States a thief was photographed by a flashlight kindled in this way, and afterwards captured through the likeness.

The level of water in cisterns and reservoirs, can be told in a similar manner by causing a float to rise with the water and make the required contact.  The degree of frost in a conservatory can also be announced by means of the mercury “thermostat,” already described, or some equivalent device.  There are, indeed, many actual or possible applications of a similar kind.

The Massey log is an instrument for telling the speed of a ship by the revolutions of a “fly” as it is towed through the water, and by making the fly complete a circuit as it revolves the number of turns a second can be struck by a bell on board.  In one form of the “electric log,” the current is generated by the chemical action of zinc and copper plates attached to the log, and immersed in the sea water, and in others provided by a battery on the ship.

Captain M’Evoy has invented an alarm for torpedoes and torpedo boats, which is a veritable watchdog of the sea.  It consists of an iron bell-jar inverted in the water, and moored at a depth below the agitation of the waves.  In the upper part of the jar, where the pressure of the air keeps back the water, there is a delicate needle contact in circuit with a battery and an electric bell or lamp, as the case may be, on the shore.  Waves of sound passing through the water from the screw propeller of the torpedo, or, indeed, any ship, make and break the sensitive contact, and ring the bell or light the lamp.  The apparatus is intended to alarm a fleet lying at anchor or a port in time of war.

Electricity has also been employed to register the movements of weathercocks and anemometers.  A few years ago it was applied successfully to telegraph the course marked by a steering compass to the navigating officer on the bridge.  This was done without impeding the motion of the compass card by causing an electric spark to jump from a light pointer on the card to a series of metal plates round the bowl of the compass, and actuate an electric alarm.

The “Domestic Telegraph,” an American device, is a little dial apparatus by which a citizen can signal for a policeman, doctor, messenger, or carriage, as well as a fire engine, by the simple act of setting a hand on the dial.