EXPLOSIVES—0’HERN, Ort 
Method of firing.—For electrically-controlled mines a continuous 
insulated cable extends from a mining casemate in the fortification 
on shore to each mine in the adjacent waters. Observers are main- 
tained to watch for the approach of hostile vessels and to plot their 
position with respect to the mines. The electrical system is usually 
so arranged that the striking of a mine is automatically signaled to 
the operator on shore, who may then fire it at once or after a few 
moments’ delay in order to allow the hostile ship to get well over it. 
The electrical method of control permits the safe passage of a friendly 
vessel. Mines which are set adrift or are planted at the entrance of 
harbors without providing electrical control from the shore are 
equipped with a firmg mechanism or fuse which operates upon the 
shock of contact with a vessel. This has the decided disadvantage 
that it functions equally well whether the vessel be friendly or 
hostile. 
DEFENSIVE MINE SYSTEM. 
The sketch herewith shows the general arrangement of a defensive 
mine system covering the entrance to a harbor. Concealed and pro- 
tected in the fortifications is a mining casemate (C) which contains 
the electrical generators, switchboards, and instruments needed in 
the service of the mines. The mines are planted in small groups 
for convenience of cable service. 
In the sketch (fig. 8) each small circle represents an individual 
mine. The arrangement of the mines is such that a hostile vessel 
can. follow no reasonable course into the harbor without encounter- 
ing one or more mines. Gaps forming a more or less tortuous chan- 
nel are sometimes left through which friendly vessels can be con- 
ducted by guide boats. In order to prevent the enemy from remoy- 
ing the mines or destroying them in position and thus clearing a 
channel, rapid-fire guns are usually mounted to cover the mine 
fields and prevent the sending in of small boats or tugs to accom- 
plish this purpose. Searchlights are provided to illuminate the 
mine fields and prevent such action under cover of darkness. 
TORPEDO. 
A torpedo is merely a mine carried at the forward end of a self- 
propelling vehicle. The motive power is usually compressed air 
stored in a tank under very heavy pressure and supplied to two 
propellers by means of a compressed-air motor. These propellers 
turn in opposite directions in order that the torpedo may not be 
thereby turned over. In order to secure greater power, arrangements 
are made to heat the air by an alcohol torch in its passage from the 
tank to the engine. The torpedo is discharged from a launching 
tube mounted on a ship’s deck or built into the ship below the water 
